I'd put together parts from newegg for you, but I am at work and using IE6, which is a pain for such a thing (due to the lack of tabbed browsing). I'll look tonight when I get home (where I also have Firefox).
I'll give an idea of what you'd need.
skt939 motherboard with integrated video and audio.
1GB of RAM (RAM can be used pretty heavily depending on how many tasks you give your server).
A decent size HDD (250GB should do)
DVD combo drive
There is no need at all for anything other than onboard video. A mid-range vid card (7300GT or 7600GT) is for GAMING, not servering
Also, as for software, you can use regular Windows XP, depending on what kind of server you want to make. Game servers run nicely on XP, as well as file servers.
For a webserver, you'd probably want to run Linux. I'll let the Linux-gurus explain why (if you need more info on it).