I personally don't think it's worth it. The 6600GT is still a good card. AGP is being phased out and the 6800GT AGP is hard to find, and overpriced. If you went PCI-E, you could get a 7800GT for $299. I would wait and save some more money and get a new mobo and maybe processor so that way you can get a PCI-E card, which will last you a lot longer and you will pay less.
AMD 64 3200+ Newcastle@ 2.2Ghz
1GB (1x1GB) OCZ Performance Series
ATI X700 Pro 256MB OC\'d 475/525
OCZ Modstream 520W
Samsung 914v 19\" 12ms LCD
SB X-Fi w/ Logitech Z 5300e