it'll probably be around the spec needed to actually play the game, a dedicated server doesn't need to be immensly powerful, as its only running code and not the actual game itself (if that makes sence )
if its truely dedicated, and if you're going to do it through xp.. you should down all system services the game doesn't need.. get lots of ram.. a moderate cpu.. and you don't even need a video or sound card..
i'd recommend a Linux box over xp
plenty of mem
best cpu you can get (but like SickBoi says, moderate, it doesnt have to be an o/c'd 3.2ghz chip )
the UT2003 dedicated server i think is about a 500mb install so you dont need a 120gb HD either.
But again, it all depends on how many players you are wanting to 'host' at one time. I think you should try asking / e-mailing someone over at the official site...
Yes, I believe you would be able to join in with your main machine.
If I was to build a dedicated server for hosting a game...
At least 1.6 GHZ Processor ( Go with AMD, and with Proper Cooling ) if you choose Intel, no lower then 2GHZ
Memory wise, I would have what I have now, PC2700 DDR RAM, 1024MB.
Graphics card, anykind, because you won't actually be playing the game on it.
You don't even really need a sound card.
HDD wise, I would go for about 80 GB with an 8MB buffer...
If you have a dedicated 100Mbit connection and a ton of users that spec could be ok, but for DSL or cable gaming a PIII 500 with 20 GB disk and 256 MB (maybe 512) RAM should suffice. I mean, it's just the server bit. It couldn't take that much.
500mhz seems a nice speed, i'd recommend the 512mb over 256mb, just to make everything go that bit smoother
theres always chance to upgrade anyway, so you could get a low spec server machine then simply add to it later if it doesnt suffice.