As far as I understand, 3D rendering is heavily CPU intensive, and doesn't really require GPU power at all. As I'm sure you've noticed, it always uses a helluva lot of RAM
As you haven't given any guidelines (please view this thread, reference for next time:
http://www.tomshardware.com/forum/261222-31-build-advice ), I'll just tell you what you'll most likely want to look for.
CPU: Quad- or Six-Core. Intel makes very nice quad cores (Core i7 series), while AMD makes some nice budget-oriented Quad cores as well; AMD makes
very affordable Six-core processors as well. The choice is yours!
RAM: Lotsa, lotsa RAM! I'd go for at least 6GB-8GB, maybe more depending on how large these renders are gonna be. Again, you'll have to make the call.
Motherboard: Not overly important. As long as it has the appropriate CPU socket, and has the features you want/need, it's good to go. I usually pick motherboards mostly by ratings.
Video Card: As I mentioned, not overly important. Something cheap and simple will be fine, just for connecting your monitor, and displaying these lovely renderings.
Hard Drives: You'll probably want something that's fairly fast (Western Caviar Black or similar). You may even consider a RAID0 setup for even faster speeds, or even and SSD for the OS/Program drive.
PSU: Get something reliable. Corsair, Seasonic, and Antec are the best brands, imo.
http://extreme.outervision.com/psucalculatorlite.jsp That's a PSU calculator; it'll help give you an idea as to how much power your system will draw. Make sure to give yourself a bit of headroom.
Case, DVD Drive, etc.: Not overly important. 120mm or larger fans will help you move a bunch of air with less noise, if that's an issue. As long as everything serves it's purpose and you're happy with it, go for it.
Anyways, I think I pretty much covered everything. Make sure your OS is 64-bit, or else you can't use more than 3GB or RAM.
I know I just kind of generalized here, but hopefully I've helped give you some insight. If you have more questions, please ask!
Good luck