WinXP will run the games better because it doesn't have so many services running in the background. Otherwise, DX9 runs on 2003 fine.
Why do you need 2003 as the base OS? Virtual PC 2005!!! If you want to learn, virtual hardware is the way to go, trust me. You can build your whole networked environment (or multiple environments!) and run it on one PC. Just get 1 or 2 gigs of memory if you're gonna run it. For cost effective purchasing of Microsoft software for learning, google Microsoft Action Pack. $300 gets you an obscene amount of software - 10x XP Pro, Server 2003, Exchange, SQL, VIrtual PC 2005, Office Pro, Front Page, Small Business Server Premium, and much much much more.
For the video card, I would go 9550 or 9600 Pro. XT if you don't spend much more than $100. Over that price, and you're too close to an NVidia 6600, which is significantly faster.
About Jam's comments concerning 512M of RAM, if you're gonna run 2003 Server as a base OS, go ahead and get one gig of memory. Take it from an MCSE.
But again, I'd highly recommend you check out Virtual PC 2005 (or VMWare) if you need to learn. If you're gonna build a virtual network, again, 2 gigs of memory would be the best way to go.
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