Linux will run great on just about any hardware you've got. The only reason to choose between IBM, Dell, and HP would be if you were planning on support contracts. If you were, I would say Dell for cost reasons. If you were looking for a 64-bit platform, then I would go IBM, but you'll pay a lot for a home system.
What are you planning on running on the server? If it's just a file server, then I would check out e-bay and buy a Dell Precision workstation, really solid stuff. I personally run Fedora Core 3 at home on a Pentium II 450 as a file server and it runs fantastically. File transfers, deletions, and copies run about 20% faster than on Windows.
The best way to choose a distro for Linux is to download all of the ones that you are interested in and install them all. You'll make a decision pretty quick as to wich one you like the best after using them for a bit. Keep in mind also that Redhat is now commerically supported and will cost you money to support. SuSe's software updates are free, but the install is time consuming because you have to do it over the Internet. I have no experience with Mandrake. There are also directory structure nuances that are different between distros that might throw you off at first as well.
I hope this helps you a bit, and congrats on NOT using Windows as a file server at home!
Andy
