Generally 1.2GHZ or so P3's, a gig of ram, and IDE raid. These work great for a ton of smaller sites, or a fair number of medium to large sites.
I'm getting a few more servers next month, dual P4's and SCSI RAID, etc.
If you are looking to go dedicated for your threads site, I wouldn't see buying a dual P4 with top of the line everything else unless you have a few hundred members online at all times and and you are seeing a few thousand posts per day. I tested my site out on two different servers before I made my final decision, and with 50 or so users always on and 1000 or so posts per day, the cheaper P3 with IDE provided about the same page generation times as a test dual P4 system with SCSI. And that system was over $200 more per month.
So the best idea is to really analize your needs. Project how much growth you are expecting to get, etc. Sure it would be nice to jump right into a top of the line server so you wouldn't have to upgrade for a few years, but when you think about the extra few hundred dollars per month compared to a completely fine less powerful server it could cost you much more in the long run
Threads is very efficient, so take advantage of it and don't bust your wallet
