You don't say which CPU you are getting.
If it's the E6400/6300, for sure keep the P5B, as they have been OCing very well when combined with the P5B Deluxe, as in over 450+ in some cases. (Much better than you could do with the P5W DH.)
The verdict is still somewhat out on the P5B + the 4 MB cache CPUs, but even still, i think it will do fine.
If it is indeed an E6600/6700, then if you're worried, you could exchange, though i'm bet that they would OC just as well on the P5B.
Hard to say though, since not enough people have tried that combination yet.
Check this out though:
http://www.xtremesystems.org/forums/showthread.php?t=111983
& this:
http://www.xtremesystems.org/forums/showthread.php?t=110121
& here:
http://www.xtremesystems.org/forums/showthread.php?t=112077
Based on the results from the first thread, i'd personally keep the P5B.
I think as the bioses mature even more, 965 will become a better choice than 975, since it's actually designed for C2D.
Only time will tell.
I think that regardless of what you decide, you'll be able to OC quite well.