well, the timing of ram is been tweaked and hard to overclock too high unless change the timing!!! i saw some companies tweak their 7900gtx not as tight as original to give user some overclock...
My vmoded 7800GTX512
the "original no vmoded" 7800GTX512 is very hard to reach 650 without artifact... also i tweaked the timing of this ram. without vmoded, my seting can only go up to around 1700 (850), very very tight timing, that is why everyone saw my vmoded 7800GTX512 overclock before, the speed of the ram is only overclock to 1950ish
let's see 7900GTX vs 7800GTX512 vmoded
650/800 vs 650/800
3Dmark06 is set as
1680X1050 plus anisotrpic 16x
in fact, i feel only the 90nm core is smaller than 120nm, there are not diffierent under same speed!!!
there are only few non-vmoded 7800GTX512 can do 650 without artifact!!!