Why change then?
I have the P35C DS3P (or whatever) and 4gig of Geil DDR2 4-4-4-12 ram.
(and a Q6600 G0) and the PC overclocks fine at 3gig. I havent tried to go any higher yet, but i'm certain it'll go there, it's running very comfortable at the moment.
I think the question you should be asking is what cooler to use.