If its a rather new board (about 2 years), its probably going to be an 8x AGP board.
I got a Radeon 9800PRO for my motherboard which is an 8x card, and I'm happy with it. It runs all games I throw at it, at decent settings too. Though the motherboard I had at the time was 4x, but it still ran fine, but maybe not as fast, and a few effects in games were cut out because they took more power than the 4x board could provide (For instance, Need For Speed Underground's motion blur effect). It works great now though (8x)
Anyhow though, I wouldn't advise you to get that card now, but its an example.
I advise you get the 6600GT, yes. A very worthy card, and it'll do you well
Make sure you get the AGP version, connect it up, and within bios (del at bootup), disable the onboard video, and your sorted.
I hope this helps.