Considering Windows 7 will be out later this year, there is no point in buying a brand new computer and sticking a 7 year-old OS on it. Unless you're running some really old games (games for Win 9x), there is no reason to go with XP. All newer games will run on Vista / 7 and they will run just as well or better than on XP.
(If newer games come out that only run on DX-10 or higher, they won't be able to run on XP at all)
------------------------------Desktop: Windows 7 Professional 64-bit; Intel Q6600 CPU; E-VGA 780i SLI motherboard; E-VGA E-GeForce 8800GT; OCZ Vista 4GB dual-channel kit; Ultra X2 750W power supply; 2 x Seagate Barracuda 7200.11 500GB in RAID 0. Laptop: Acer Aspire 8730-6314;
Reply to Zoron
I say XP b/c it takes much less resources, less background operations. the only thing that vista is good for is DX10. but you can run DX10 Crysis on XP of course by going into the cvar files =P