Why would you say to go with XP when I have a DX10 video card ?
You have a 2600XT. For gaming, Vista performance won't be very good so XP-64 is indeed a good option. If you had a more powerful card then Vista would be OK. But with your HD2600XT under Vista it would lower your performance.
Just because it's a DX10 card it doesn't mean you should go with Vista
In my experience server 2003 makes a poor gaming platform. Largely due to it, for some inexplicable reason, disabling advanced graphics capabilities when it is in use as a domain controller. Maybe it is to encourage you to not use a server for software better left to a workstation where a crash won't take down the network.
Regardless it is not a very good performer even without a domain running on it.