Reason not to get the PPU: The 8800 GTX Has one on it (it's a 512mb card, with a 256 mb PPU)
Reason not to get dual GTX: SLI doesn't mean you're going to be 2 fold better, it's a slight increase, it's more for overclockers.
If you were an "Extreme Overclocker" go for it (you're buying premade, which pretty much rules that out), but you say you're an Extreme Gamer, getting 1 top of the line (yes, the 8800 GTX is the best you can get for gaming at the moment) will be plenty
Also I'm assuming you're a WoW player...lets get real, nearly anything can run that game. But I'm gonna give you the benefit of the doubt and say that this rig is for future games... One 8800 GTX is still plenty.