I think there could be kinda three things that can happen, and according to various threads they are all equally probable (you could make a pool to see):
- since the cap is @4GB (2^32) then 2GB for the RAM and 2GB for the GTX280s should be just fine
- more often when putting 4GB on a 32bit XP, only 3.5GB are accessible from which you subtract the quantity of Vram (ex in my case from 4GB 3.5GB-256MB makes for 3.25 accessible system memory); this would leave you with only 1.5GB of adressable memory.
- it may not work at all in rare occasions; if the OS sees less ram than the actual Vram it may refuse to boot: say you have 2GB system memory and somehow 500MB are lost (4=>3.5 conversion makes for a 2=>1.5GB accessible), but you're with 2GB Vram. It may happen, even if rarely, the OS to refuze to boot because of driver or who knows what conflicts that are never documented; there are 2 threads I know of on OCN on this.
Anyhow, hope you're in the first category, it would avoid wasting any resource.