The GT has a small edge (~10%) over the 800 Pro.
Compare the best prices you can get for a decent brand card and see which is more cost effective. The top end cards are too expensive to risk second rate manufacturers like PNY, etc to save a few bucks.
EDIT:
I got curious and checked. Both a little over ~$400 which means $450 with shipping, tax, etc. So the GT would probably be the better buy. With hthat kind of price tag my 9800 PRO is looking real good.