Honestly...Fraggle.
The only thing you'd NEED is the mobo. A 6800GT, or any 6800 for that matter plays any game thats out today--and tomorrow, fine.
Mach II (Phase Change) I don't think is worth the hassle for a chip you aren't going to have much longer.
Actually, ya know what. I'd get a 7800GTX.
That is future proof.
For the main system itself (CPU, RAM, MOBO) I'd wait another year...because AMD is releasing their Socket M2 I think it's called, and who know's what Intel is cooking up...thats what I'm doing at least.
I just think it's a waste to dump tons of money on a system that's only gonna give you a little bit more performance. Also, that board isn't a DFI. You'd loose the 4v RAM, etc etc.
DFI is coming out with an ATi Based board soon...so I'd sit tight for that, IF you insist on upgrading.
Concluding my statement, if you look @ the 7800GTX....then think about the R520, what happens, if you get the ATi motherboard, 7800GTX, then the R520 comes out...and OWNZ the 7800 series.
Then you'll be screwed cuz you just dumped $600 into a card that just got raped.
So...while all those scenerios above have pro's and con's, I think you should just wait.
With all the new technology being released, the last thing you want to do is get screwed.
Wait a year or a little less for the new Intel & AMD processors, and the new nVidia and ATi video card's, and the new Chipsets, before you get anything.
EDIT: whoa that board does to 4v Vdimm
