I don't know....
But traditionally with Quake stuff I know that it generally takes less hardware to run a server than it takes to play the game with nice graphics.
Most people that brag about their massive powerfull servers, are either full of it or play the games on the "dedicated" servers....
My suggestion would be:
for maximum performance: run Linux... do a minimal installation with no gui and no bells or whistles, configure a custom kernel that has just your hardware support built in.
and get the server
files.
However more than likely you are a windows user, so do a clean minimal install of w2k.
For the hardware:
Find your buget. The more money you have the better the system you can afford, but it's pointless to talk about getting the new 2.8 gig processor if you can't pay for it...
The most important thing is you connection to the internet. this is were the big bucks get spent! I trust you mean kb with a little "b" (for bits) and not "kB" (for kiloBYTES). 320 kb spread among 20 users is only 16 bits between them, thats less than a third of the bandwidth available from a 56k modem (56k modem is advertised to provide 56kb's of bandwidth or 8 kB), If you'd be sure to limit bandwidth to 32kbs for each user I'd gues you would get about 8 users before you would start getting some major lag....
find a happy balance between proccessor speed and memory. I am guessing, but any thing over 1.5 gigs would be overkill. I'd rather have a 1000 megs of ddr sdram... If you spend all your money on processor and have little ram, then your server will have to use swap files on the hardrive and you can kiss your performance goodbye.
a decent motherboard is important, but buy one that is lean on features and high on performance. No point to having a high-end top of the line motherboard unless you are overclocking...
Forget wasting your money on a nice video card. Just use a generic one, or a old used one lying around. No point in buying a geforce4 for a computer you will never use it on.
Forget nice harddrive, a 1 or 2 gig is enough. A faster hardrive may increase read write performance, but you want your OS and Server programs to stay up in RAM were they are 100X faster than any hardrive....
soundcard?!? what soundcard!
However if I am right about your bandwidth, I'd say you could go with a on 400 pentium system with a memory upgrade to about 516 megs and run win98 (and reboot every once and awile to recover memory leaks!) and be fine and dandy for only 8 clients. It would be good for a clan meeting place or a small public server that you could invite freinds to. go to a nice wolfenstein fan site to find the facts though...
just my 2 cents.