Well, looking at the specs, you'll likely be stuck to low/medium detail, XP will still likely be fastest option due to your lack of system ram for the mentioned games, although if you jump to Win 7 with its better optimisations and Dx10, the gains may not be that noticeble due to the low end hardware you have, Dx10 in itself may glean a few nicer looks but nothing earth shattering.
Generally speaking for Win 7 as a gaming platform you really need 2gig ram and dual core cpu teamed up with a nvidia 8800 (or higher) or a ATi 4650 (or higher) before you'll notice real the advantages over XP, since multicore cpu support on windows 7 is optimised compared to XP's, in my years of experience.