Know how many players you will be able to support
shouldn't need to much speed, Celeron 500 w/128-256MB of Memory.... and stupid HD.. Linux would be best.
If your running bots i think that would put a big load on system and you'd need faster. But i think Celeron 500 should be enough for the connection you probably got, in Q3A im pretty sure it could hold at least 24, mabe 32 players under linux; You'd have to have like 1.5/1.5MB DSL to support 32 players 😉. Not sure about how much Q3 actualy uses conpaired to CS (my game). i don't think it works too good overclocking in linux because it uses the CPU so much better then windows it heats it up more.
I don't know if you want to go excesive and have it waste alot of potential CPU power and money, or just go the cheap way and get what you need right now... I think Celeron 500 should be more then enough as to what your connection is.
But it shouldn't cost you too much, Celeron 500 is pretty cheap, mabe get a dual MB so you can add another one if you needed more power (For what ever reason), 128MB should be fine, might want to go with 256MB, again im not sure how much resorces Q3A really takes up. HD just a cheap 4GB drive for like 40$ or something =-) Once its started it never really does anything with the HD anyway, mabe write logs but thats about it. Cheap S3 Video card or something, mabe no monitor, I never use my linux box's monitor, only have one because i already had the monitor laying around.. Just Telnet/SSH into it.
Good luck