Quad core for XP is not necessary. Though there are fixes, some games really don't like multi-core CPUs (for example Need for Speed: Underground 2, Fallout 3), but there are fixes, otherwise the C2D E8xxx series have plenty of power for XP era games.
The Q/P/G35 and Q/P/G45 chipsets are good.
Graphics card compatibility is overall good over the XP era, you can go with late-era cards for performance. Some early era games have issues with late-era graphics cards (for example Doom III not detecting VRAM correctly, Republic Commando crashes past GF7000 series cards), but there are fixes. As for speed, there are certain games that need a lot (Mass Effect, World in Conflict, Crysis). I wouldn't go below a GTX650 Ti or equivalent. This one also offers ShadowPlay support if you want to record gameplay footage without other equipment, but negligible performance loss. Also, a lot of games past 2008 support PhysX which tanks performance on ATI/AMD cards.
You probably also want an EAX compatible sound card.
Speed sensitivity is no longer a serious issue in the XP era, pretty much everything can be solved with simply turning VSync on.