So, I run 4 game servers. Nobody can tell you what you need to serve games that aren't out yet. Some games are hogs if they're poorly written (and some still are poorly written, especially mods)m but this is only really important if you want to have multiple games running full on the same box.
I've found a single 3 GHz-ish P4 and 1 gig ram to go surprisingly far... I was so impressed I bought another. And it'll only cost you ~$500. On a single P4, I'm running BF, BF:V, ET, UT2K4 ONS. I've never saturated the processor, and still have ~200 megs ram free. When these are full, ET takes 10%, UT2K4 ONS (which takes twice that of other gametypes) takes ~30%. I don't have info for the BF games, sorry.
I will echo phreaqe's statement that these people are overstating the needs for a server. Yes, you have $1000 and yes, you can spend $1000 and get bragging rights (omg dual opteron! omg fancy 64-bit AMD!) but a single CPU solution works just dandy. I've yet to try the Athlon64, but it'll be next, eventually.
It is highly unlikely that a 3GHz server would be unable to host any game coming out in the next year (or two?). Your only potential limitation is if you want to run multiple servers that are full simultaneously, or if you want to run game sizes way, way larger than intended (example: 2K4 Onslaught is usually in the range of 8v8, but try 16v16 and it'll choke every processor out there -- the CPU demands begin to increase exponentially).
It would be helpful if you could tell us how many people total would be using the server at the same time...