You don't really need much to be honest. It's more on the internet connection side I would worry.
If you just want the one server, a Pentium 4 2GHZ with 2 gigs of ram will run a server very easily. It just depends how many FPS you are running it at. But chances are you should be fine.
I have heard that at default FPS you need 6kb/s upload per slot for it to be stable. It may increase per FPS but I'm not SURE if it does or does not.
FPS has something to do with the speed of the server, but I'm not sure exactly how I can explain it in detail.
So for example, a 10 player server requires 60kb/s upload, however, if you want to be able to surf at the same time, please keep in mind that this is is no different than running uTorrent at 60kb/s nonstop (if it's constantly full) and if you are on DSL, it will substantially slow down your internet connection's responsiveness.
If you know how much upload you have, make sure there is some room for general internet tasks. For example, if my max was 80kb/s, I might run a 10 player server, with the 20kb/s made for my general internet connection.
If you don't know what your max upload is, you could run a speedtest at
Speedtest.net - The Global Broadband Speed Test I believe, but alternatively you can just check with your ISP..
To save you some time, if you're using Bell or a Bell DSL reseller, you most likely have roughly 80kb/s upload (on their 5mbit/800kbps plan)...
I hope this helps.