Windows 7 or Windows XP?

I am putting together the following rig from a mix of old parts I have lying around and a gfx card and hard drive I have purchased on his behalf.

Athlon 6000+ X2 @3GHz (Dual Core)
2GB DDR2 500MHZ Ram
AMD 1GB 6670 gfx card.

Since his laptop packed in, this PC is mostly to get him back on the internet and will also be used as a multimedia PC, will be probably used to play some old school emulator games, and possibly a bit gaming with low demand PC games such as Pro Evolution Soccer 2012 or FIFA 12.

I know it is a piece of shit and any upgrades are out of the question, but my dilmena is, do I install him Windows 7 or do I go with XP. Obvioulsy W7 is a much more pleasant user experience compared with XP, but since there will be a bit of low demand gaming done on it, I am thinking that Windows 7 would throw too much CPU/RAM overheads into the fray, possibly losing crucial frame rates compared with how they might run in XP.....and possibly, XP will run noticeably nicer in terms of everyday use?

Is there even such thing as DDR2 500mhz??

Get him some 800Mhz DDR2 or 1600 (might help with overclocking if you plan to do a little) that 500mhz is gonna seriously effect overall performance