I probably would've gone for 4GB - 2x2GB for dual channel (probably not that much of a speed boost, but it's practically free).
4GB doesn't imply you have to have 64-bit, though there's little reason not to use it (basically 64-bit gives a speed boost at expense of some RAM).  That's not old hardware - I've been using XP64 on my old machine fine (AthlonX2 4200+, nVidia 7800GT card, 1GB RAM).  But meh, 32-bit works, so who cares?

Good build otherwise :P