What kind of modeling are you going to do? How much real-time rendering? What programs?
Sometimes it helps tremendously, sometimes it doesn't make any difference.
There is a V4800 that is VERy good for the price for workstation cards at the moment. 1 GB gddr5 mem, good clocks, 170 bucks.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814195096&cm_re=ati_firepro-_-14-195-096-_-Product
If you plan on doing some gaming as well, then definitely get a gaming graphics card, and then use it for 3d modeling, it should be able to do almost anything.
There are some benches floating on the internet on this subject, and if you are ONLY going to do 3D modeling, and you rely on efficiency (job-oriented), and you real-time render (graphically)really LARGE models in space, then get a workstation graphics card. The differences between them can be found on the web, and with the same chip, etc. the workstation was about 30% better than its gaming counterpart (the most extreme difference, generally it was about 15-20%) in rendering SW models faster. Which, depending on your application, can make a huge difference. Waiting 5 hours as opposed to 7 hours, etc.
For the price, the v4800 can NOT be beat right now. Only problem is, if you need more power than that, look for a 600 dollar card, because there is no Crossfire compatibility (two cards running at once in case you didn't know) on the firepro series right now, as far as I understand.
Hope this helps.