My league is exactly like this with the exception that an undrafted player can be kept as an 8th. I can tell you the best strategy by far is to try to keep later round players that will still be in your starting lineup. If you can keep players beyond the 6th round (that are good enough to be every week starters), then you will be drafting backups while other guys are still drafting their starters. that will allow you to make some killer package trades during the season. Alot of guys havent figured it out and they keep good players-Peterson, Manning, etc, but it costs them a 1st round pick. If you can keep a late round guy like benson, then you get Benson AND a 1st round pick. So you have the potential to get Benson + another stud like Fitzgerald instead of Manning and whatever scrub is left in the late rounds.
Here's what i think:
QB: Matt Schaub (8th)-good value, not surea bout keeping a QB with the other choices available.
RB: Cedric Benson (7th)-Yes!
RB: Knowshon Moreno (10th)-good value, but im not sure he's an every week starter
RB: Ryan Grant (4th)- a little early. you coudl draft him in 2nd or 3rd if you want him
RB: Matt Forte (1st)-he is a definite NO-agreed
RB: Beanie Wells (8th)-i like the value and potential, id consider him, pending RB/QB/WR changes in ariz.
RB: Chris Jennings (undrafted)-no
WR: Roddy White (3rd)-no, good player, not good value
WR: Wes Welker (5th)- i like this value. id consider him
WR: Dwayne Bowe (6th)-good value, terrible team
TE: Brent Celek (undrafted)-dont waste your keeper on a TE
TE: Fred Davis (undrafted)-ditto
Id rank them like this for now: Benson, Wells, Welker, Schaub. (depending on your league settings and how the situations change in the offseason) if your league allows trades, thats a great way to stockpile picks or move up a few spots in the draft. Some teams probably have nobody good to keep and it should be worth a mid-round pick to them to get one of these guys.
Edit: You also need to consider where you are drafting, and what positions/players will be available. If you think manning or Brees will be a good value at your pick, then i definitely wouldnt keep Schaub. likewise with keeping 2 RBs if you can get a better stud Rb with your 1st pick. A mock draft will be very helpful. (my league does a serpentine draft, so that might affect your keeper too, there's a big difference between picking in the early 2nd and late 3rd but not much between a late 2nd and early 3rd.)