For starters, check out BOXX. They make machines dedicated to rendering called the
renderBOXX. Then from there, if you wanted to go the build-your-own route, you could base your specs on the BOXX and scale up or down depending on your budget. Definitely don't go cheap on the CPU's, though. For instance, you could probably buy lots of single-core Pentium 4's clocked at 2.8 ghz, but four of those won't render nearly as fast as a proper Q6600 clocked at 2.4 ghz.