I wouldn't. You could dual-boot, but I don't really see the use of running winXP or 2k on it. If I was to run older games I'd want to run them all out graphically and this machine is best suited for top-of-the-line win9x stuff performance and quality wise. I'd say you made a good OS choice. Since 2001-2008 retro gear is so cheap right now, building a setup capable of running these games maxed out is relatively cheap and easy, so why not do it properly with an AM2/LGA775 PCI-E machine?
For me winXP gaming means stuff like Doom3, Quake 4, Black and White 2, Stalker and C&C 3. While the first two would run well enough at 1280x1024 + AA on your rig, cranking graphics up to 1600x1200 (or 1920x1080) with 4x AA requires a way faster machine for over 60 fps - that's 7900GTX SLi / 8800GT / 2900XT territory. I know because I tested the game on my 3800+ / GF 6800 rig and performance at those graphical settings is lacking.
Great rig BTW, it's similar to my win98 overkill build: Win98 Socket 939 Voodoo 2 SLi Build! (a.k.a. Glide Overkill)