Still there is much dependant on what kind of games you will be running, what the maxplayers will be for each game, how many FTP users you will allow, and what kind of rate you will allow players to connect at.
I have run 2 Q3 servers on my PII 450 w/ 198 MB ram with no problems whatsoever when it comes to the "running" of the games. As you properly identified, the problem will be with your bandwidth.
On my cable, I would run a bandwidth monitor and I saw that as soon as the transfer rate (total) would hit about 450k, I'd have problems. Since I run almost nothing but custom maps, this is a problem only when people are getting those maps. Normally, the 512k line you were talking about would be fine to run the 2 servers, however I forsee problems if you plan on running a FTP server, and if the games will have anything requiring large downloads. My solution (since I can't increase bandwidth) was to limit the rate at which people can connect to the server. By reducing it from 25000 to 10000-15000, you can have many more people "downloading" (I don't mean playing the game, I mean downloading big files). This rate reduction, however, comes at a cost. Figure that if you half the rate, it'll take twice as long for people to get the data. In Q3 (the only game I have experience with) If the max server rate is set to 50000, people will download around 25k, at 25000 it drops to about 12k... and so on...
I say, that if you can, and if you can afford it, get the most bandwidth possible. As for the server being dual CPU or not, the fact that you're running Win2k will allow it to see the 2 CPU's, but I don't think you'll see much of a benifit from it (if any). 256 mb ram and a decent CPU (500Mhz+) should be fine as long as you're running DEDIACTED servers ONLY. If you plan on playing games on that box at the same time, that's a whole 'nother story.
Hope this helps.....
Legrand [LN]