IMO, best case is he's a legit #5 next year. LJ, LT, Alexander and Portis aren't going to fall of the face of the NFL this year, while Bush may be great, I highly doubt he'll do better than those 4. And they aren't exactly elderly.
I wouldn't put it past those guys. No player is a lock. In the past few years, elite guys like Holmes, Faulk, McAllister, Ahman, and Culpepper have seen their values take serious hits. I definitely see some risk with the flavors of the year in 2006. Alexander has a lot of carries under his belt and is coming off a fairly fluky 2005. Tomlinson has a lot of mileage. Johnson has never been the starter for a full season and he plays with an ancient supporting cast.
Portis is solid, but hardly spectacular.
Sure, there's risk on every player, but unless you want to start predicting injuries, I don't see where these guys, who have been consistently productive, will fall off this year. There's more risk on LJ and Alexander, but that risk is still significantly less than with Bush. Spare me the "mileage" talk with Tomlinson. How many hits does he actually take vs. other top RBs?
On Portis, you may not call it spectacular, but
top 10 in yardage every year, 3 out of 4 in the top 5; along with 2 top 5 TDs and another in the top 10, doesn't impress you, be prepared to be disapointed with Bush.

Deuce looms large when healthy. Reggie is a weapon that will be utilized in situations where his skills fit best. Reggie won't be a feature RB. Took Bush with the first pick of my dynasty rookie draft so I'm no hata. He'll be fun to watch but a word of caution. The NFL has not seen a player with his hype enter the league since Kijana Carter. Kijana is a fitting example for anyone arguing the top RBs could crash and burn. Reggie will be in a RBBC and of the major components of determining dynasty RB rankings, situation is HUGE.
Dynasty RB factors
1 - Health
2 - Talent
3 - Situation
4 - Age
Reggie could rank higher with PPR scoring, otherwise his situation will dictate his FF production.
This is how I have dynasty RB rankings in non PPR leagues.
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1 - Tomlinson San Diego Chargers
2 - Larry Johnson Kansas City Chiefs
3- Shaun Alexander Seattle Seahawks
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Elite class, no explanation recuired.
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4 - Steven Jackson St. Louis Rams
5 - Clinton Portis, Washington Redskins
6 - EDGE Indianapolis Cards
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Second tier of clear cut stud RBs (see Stephen Jackson ranking in Kevin Jones ranking)
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7 - Cadillac Williams Tampa Bay Bucs
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Caddy is in a seperate category. He exploded onto the scene last year but didn't hold up so durability is an issue. Suspected last year that Caddy would have durability issues since he never carried the load in college and split carries in a RBBC and doesn't have the largest frame, much akin to Reggie.
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8 - Ronnie Brown Miami Dolphins
9 - Kevin Jones Detroit Lions
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Ronnie was the most talented back last year but lacked the killer instinct to take over as the lead RB. His situation would be a RBBC for as long as Ricky was around. Now he has no competition and he should step up. Recall a guy who used to be with the Browns, Kevin Mack. Ronnie's situation is much like Kevin Mack. Mack came in to the Browns from the USFL, a big 6' 232lb 4.5 forty RB but he lacked confidence. Browns head coach Sam Ratigliano saw the lack of confidence and didn't let him fade into the background but instead told him that he was going to be his feature RB and once he was forced into that role he stepped up. Saban last year saw the shaky confidence of Brown and made public mention of it. Anyone who read that situation knew Ricky was going to be seeing lots of touches last year but now Saban has Ronnie Brown and hes talented enough to step up bigtime.
Martz is going to use KJ ALA Marshall Faulk and he says that KJ has caught handled everything he's asked of him. If you recall how Martz used over twenty different variations of the Rams stapled running play called 'THE FLIP' where Marshall would do that patented 'STOP STUTTER STEP' then take the ball and go. One play with twenty variations. The defense ALWAYS seemed to get caught off-guard with the FLIP. Absolute GENIUS at work with Martz. Marshall was solid but not ALL-WORLD till he hooked up with Martz. The stellar success of Padre with KC sprang from the great O-line AND the rushing attack tips Vermiel picked up from Martz. Stephen Jackson is more suited to the verticle spread attack that Scott Lineham will use and Lineham has stated he's going to feed Jackson so that is why I have Jackson ranked #4.
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10 - Willis McGahee Buffalo Bills
11 - Lamont Jordan, Oakland Raiders
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I've never been particularily high on either but I do not place my gut above solid production and situations. They earned the rankings even though I just don't feel 'good' about either of them.
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12 - Rudi Johnson Cincinati Bengals
13 - Jamal Lewis Baltimore Ravens
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Both coming off injury and with a mild competition yet each is more than capable of being dominating when healthy and being fed the ball. Rudi wasn't 100% last year but is rumored to be now, expect big things from him. Jamal is NOT washed up contrary to what many folks believe/parrot. The ankle injury was the exact same one that pitcher Kurt Shilling had and it has an 18 month recovery period. He got injured in November of 2004 meaning he was not projected to be 100% till February of this year 2006. Note Jamal only had two-100 yard rushing games last year but both came within the last five games of the year when he was getting closer to being healthy and when Boller began to put together some decent passing stats. Last year Jam missed the entire off-seaon, EVERYTHING! All, OTAs, mini camps, training camp, preseason games. He was coming in off injury, he was unhappy with his contract, he had no football contact of any kind in nine months and both Ogden and Mulitaulo were injured and Zues was on his last legs. That O-line is still a question mark but Jam is back, he's healthy, he's happy/under contract, and he's got a QB for the first time in his career to keep defenses from cheating by stacking the box. Anyone who has said that Jamal is done is in for a big shock this year.
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14 - Reggie Bush Saints
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Talent-O-plenty. The Saints will find unique ways to get him touches but they are not going to put Reggie into situations where they have a better option. Deuce McAllister's place is assured if/when he's healthy. Bush and Deuce should compliment each other but it wouldn't surprize if they posted similiar point totals in two very different roles. The caveat being that both are healthy and Deuce won't be 100% this year. Reggie has to learn pass protections and their has been no contract negotiations yet so a holdout is possible.
I bumped Bush up a few notches over, Westbrook, Julius Jones, and Deuce, because each of those players are coming off injuries and with formidable competition to tap into their touches. Simply can't rank him higher than the RBs listed above him but that is my humble opinion.