hello
actually you should consider the load you're about to have on the server, and in particular if you're going to run many scripts, like TGP and that stuff with a script that you host. infact, if you don't run such scripts, there's no need to have a CPU with many GHZ, however, the most powerful server's CPU is the Xeon right now, so if you want the best, go for that.
as far as HDDs, i would never choose for my server anything that's not SCSI and i've always had SCSI equipment, they are 10k rpm and that's more than enough. latest EIDE drives had pretty good speed anyway, but for server, nothing but SCSI.
then RAM, well ram is very important, so the more you can get, the better, i have 2 GIGS and it's doing ok.
as far as domains, well there's no actual limit on that, but remember that the more domains, the more load, so the main issue would be the bandwith, so let's say that a p4 2.4 ghz can hold up to 50 mbits of bandwith without any issues, if you're going for more than that, i really suggest you to go with a Xeon as i mentioned above, or 2 separate servers aswell.
and last but not least is the operating system. our O.S. of choice is Free BSD, never had a problem, it's as stable as a rock and also very secure. but you do need a good server's admin to configure it, and eventually to re-compile the kernel. i have custom kernel on my server customized by my server's admin. don't EVER use windows 2000 server as it has too many security holes, only reason you could be using it is the ASP scripts, infact they only work on a 2k server machine or if you have a sort of emluator. so summing it up, either Free BSD or Linux.
i pretty much summed all, maybe too much, but i prefer to be clean with this stuff, as i take care of this for our company, i know the details and why we chosen something, from our past experiences.
Actually our host of choice is
Oxeo Hosting, i really like them, they're fast and efficient. Sales/support available via icq/aim aswell.
if you have any questions as far as server's specs, don't hesitate to hit me up on icq, #132516819
have a nice day!