ok- here's my thing. i have a cannondale multisport 2000. i love it. it has 650 wheels on it. i'm looking to get some good race wheels. the problem is, when i bought my bike i didn't know much about anything. now, i'm kinda wishing i had 700 wheels. i figured it out and over the course of an oly tri, the 650's require about 1700 more rotations. in an ironman, it's almost 9000 more rotations. to me, that seems like wasted effort. i mostly ride flat courses, and my big concern this year is i'm doing imf, which is also flat. it seems like i could benefit more from 700's than 650's.
so, before i drop $$ on new 650 race wheels, is there enough difference that i should think about getting a new ride altogether with 700 wheels? i've been putting in the ot at work and saving up so i can get the cervelo p3 (at this point i'd just get the frame and transfer my components). i can get the frame and a set of race wheels for about the same price as the complete bike. there's no danger of me winning any races, but i'd like to be able to do as well as i can.
ps. i'm a bigger rider (200lbs), and someone told me that weight affects the 650's rolling ability. i don't know if that's true or not, but they said 700's are better suited for a guy my size.

Go for the 700s. It is my understanding that the only reason 650s are even still around is b/c the smaller frame sizes are incapable of fitting 700s.

You also have better tire and rim selection in 700c