Well, believe it or not I'd probably ask first what sort of system you already have, and if you find it meets your needs for rendering except for the animati9on. The reason I ask, rendering does rely pretty heavily on the video card, but the real heavy lifting I've found when it comes to animation is actually the amount of processing power you have available that seems to make the biggest difference.

When I did video animation I found that an older file server with a ton of processors actually would out perform my even my I7 with a top of the line video card when it came to producing video animation sequences in poser. I was using an IBM 455 with 16 dual core 2.2 GHZ processors for the task, worked like a charm even though the video in a rack mount file server is very limited (pretty much just standard VGA, they aren't really designed for gaming, etc..)

I picked mine up used for a song and replaced the single core 2 ghz with dual core 2.2 ghz, giving me an effective 32 cores running at 2.2 ghz to handle the animation process. Worked great. However I don't recommend this be a primary machine or as a replacement for a PC, about all it was really good for was animation because of it's other limitations. So if you have a good fast machine with a decent video card in it that's doing you fine for everything else, and just want some more horsepower for animation, this might be a viable option. If you don't already have a desktop that's meeting your other needs, I'd probably do as the others have suggested and look at a good I5 or I7 machine with a decent video card and a good amount of ram.

On the file server route, please note that an older rack mount file server is a big, ungainly beast that does suck down a lot of juice. I stored mine in the basement, had it setup on the network and remote desktop into it as needed, I'd shut it down when not in use to save on the electric bills. .