I'm not sure if you want a workstation card, as I have heard that it isn't really necessary unless you are designing games and such. However, you will need a processor and memory-intensive system to handle the rendering requirements.
Start with the processor, to save a bit of money, avoid the FX and EE series from AMD and Intel. Instead look to the dual core 4800+ or the Opeterons, as well as Intels current high-end, dual core, non-EE processor, but I'm not sure which it is since I don't use Intel. You will only gain a marginal increase in rendering times with those expensive FX/EE series, so sticking with the high-end standard lines will save hundreds and yield 98% of the speed.
Next, you will need a large chunk of memory to hold more of the rendered files once the processor is done. My friend in post-production has said it works like this: If you get 30 seconds of rendered animation with 2Gb memory, you'll get to render up to 60 seconds with 4Gb, and so on and so forth. I've heard that 4Gb isn't fully supported on current platforms, but the explanation of why is beyond my realm.
Finally, a good RAID setup helps eliminate the data bottleneck while transferring large files (IE: Multi-gigabyte rendered files) around the system. It would help to use WD Raptor drives if you can afford them, and RAID 0 would yield even further increases in throughput. Those drives are Enterprise-class, whatever that means, but they do have a reputation for durability and low-failure rates, meaning your data is more secure if you purchase the more expensive drives. However, at $250+ for a 150Gb drive, they aren't cheap, especially compared to the 300Gb/16mb cached drives available for around $100. If you do go with cheaper drives, you should really use a RAID 1 configuration because it'll be cheap and secure.
Good luck.