Well, Pooh bear!!!!
BaldEagl was right. No one seems to come right out and announce it as being the case, but from what I can tell based on research of the issue both on of the NVidia forums and across the net in general, with the advent of PCI-E based video cards, aperture based access of video memory seems to have gone the way of the Dodo.
Which means that your max available RAM under 32 bit OS is going to be (roughly) 4 GB - ~ 500-750 MB - the total video memory on whatever PCI-E video cards you have in the system. Or in other words, a pair of 8800GTX's with 768 MB each appears likely to limit you to around a max of about 1.75 GB available memory under a 32 bit OS.
I've avoided to this point 64 bit OS's other than on a few work related servers due to compatibility issues and general pain the in the you know whatedness - but what is the current status of 64 bit XP or (shudder) the other one for AH? What about other games? I don't play much else, but would hate to be hindered if I chose to if I built a new rig.
<S>!
(edit for typo's and clarification...)