Of the choices you list, I'd use XP64 Pro. It does everything 32 bit XP does and offers 64 bit computing. The old driver issues are gone now, thanks to Vista 64, and XP64 is very stable. Sure no DX10, but it will be a long time before lots of DX10 games will be out, much less required. You can also use the whole 4 gig of ram, even 8 gig if you want, though only some business apps use that much.
If you decide to get Vista, upgrade to the 64 bit edition. It doesn't cost much and it unlocks a lot of potential, besides letting you use the whole 4 gig of ram.