check out tomshardwareguide.com, they have graphics card section, when I find the link to there AGP Card reviews I will post it and they have reviewed all the cards, I can tell you this though, the 6600 is a decent card at the lower resolutions, just dont turn up the detail all the way and try to kill the card!
Just one thing though when you do get your card make sure it has DDR2 or DDR3 ram as its a big difference in proformance! Thats the only thing you should really be concerned, as a DDR1 256mb 6600 card is slower and weaker than its cousin the DDR3 256mb 6600 and the DDR3 one costs about the same or less!
I shop here, the list is in ORDER as to where I go first,
stores.tomshardware.com (comparrision shopping at its best!)
www.newegg.com
www.zipzoomfly.com
www.tigerdirect.com
www.pricewatch.com
thats where I get my stuff from when I buy, really check out the store at tomshardwareguide, they have a REALLY NICE comparison tool!