In the past month, bestbuy has had the PCI-e X1950 pro on sale for $200, and the 7900GS on sale for $150. Those are pretty good prices for either one. But for $300, you are throwing away your gift card buying the X1950GT. Buy one online for $100 less and use the gift card for a few games. If they don't go on sale and you must shop Bestbuy, the 7900GS easily gets my vote as the best for the prices you listed. Of course, after sales tax, shopping online may be the better option.
These cards would be better online options than going with that $300 X1950 pro:
Faster X1950XT $250http://www.newegg.com/Product/Product.asp?Item=N82E16814102067
X1950 pro $190http://www.newegg.com/Product/Product.asp?Item=N82E16814161060