Right now the 6600GT AGP I would go with is Gigabyte, as far as lowest price and quality. I read the few reviews I could find on the card, one was comparing it to 2 other 6600GT's and it won among those. $200 with free 2nd day shipping at ZipZoomFly:
But prices could change on others suddenly, so by the time you get your tax return, there might be a better choice. (I was going to link you to $199 eVGA, but the price shot up to $250).