Okay, so the average Tremulous server allows for lets say 22 people, but your interested in hosting 50 people. For plain system resources, Ram will be the most important factor as lots of Ram gives your computer space to process whatever the fuck your players are doing.
A good processor is also a nice idea to have, as a huge server with many players doing their own thing requires the server to calculate the physics, values, etc. Also OS comes into play when hosting a server. Servers running on Linux tend to run faster compared to servers running on Vista because Linux's fast and stable architecture is easy on the PC.
As for disk Space, if you have even a 10 GB harddrive, you're good to go. Having huge harddrive space isn't necessary, but you should have enough for virtual memory for the PC to work with in case. (600MB for Quake 3 last time I checked)
Lastly its best to have a good, dedicated connection to the internet and no other programs should be on that will take away memory usage from the PC, especially programs that uses the internet.
So heres what I think:
System Requirements
Minimum:
Pentium 3; 733 Mhz PC
512 MB of RAM
High speed internet connection (Modem, Wireless or LAN. 11 Mbps)
Optimal:
Pentium 3/4 1.2 Ghz or better processor
512 MB of RAM, 1 GB or higher recommended
High speed, dedicated internet connection (Modem connection recommended.)
So in the end, unless your like me (Who has a shitty PC that even runs Quake 3 like shit) you should be okay. But if your serious about hosting a server, I suggest buying a server hoster so they do the hosting for you without you worrying about system resources. I hope what I said wasn't utter bullshit and it helped, cheers.