It looks like everyone's helping already with the system specs so I'll just chime in on the 32 vs 64 bit debate. (64 bit vs 32 bit is up there with .45ACP vs .40S&W, i.e. they're both comprable now but as .40 becomes more popular there'll be more guns in the future for it and ammo will be cheaper)


AMD x86-64 (Athlon64 and Opteron) is still better than 32bit intel or AMD chips because the memory controller is built into the processor. While the software/drivers may not be able to take advantage of the 64bit bus, the sledgehammer/clawhammer cores have consistently outperformed top end Intel chips for the same if not cheaper prices (even in only 32 bit environments). AMD motherboards are also generally cheaper, too. See the above links from other people for Toms hardware guide and Anandtech for benchmarks.

Puting together your own PC isn't hard, as far as the assembly goes. They've got everything such that you really can't plug the wrong things in to the wrong places. The fun part is when you start installing software and drivers. I'd get Windows XP Pro (NOT HOME!!!!!!!111ONEONE) and upgrade to SP2 before even thinking about hooking it up to the internet. After that, ignore the CDs that came with your hardware and go to the manufacturer's website and download the latest drivers from them. This should save you many headaches as often bugs are found right after driver CDs are already pressed and in the box. Do not install any drivers from Windows Update (but DO install most of the other stuff there, especially the security updates). Make sure to get a good virus scanner, AVG is free and works well.

Should you get yourself into a bind, the easiest way to fix it is offer a local geek (coworker, friend, someone local from arfcom) a 6 pack of beer and/or a pizza and he/she will usually be more than happy to come over and help. If you're lucky, said geek will probably be nice enough to show you how to do everything the right way and you'll learn and be able to fix stuff in the future for yourself.

Have fun.

ETA: If you do decide to go with a major manufacturer (Dell, hp, whatever) don't look at the home desktops, look at the business models. They're genrally bundled with a lot less crap software, have XPPro by default, and tend to be made with better quality components. Sometimes they're even cheaper, but that's hit or miss.