My GeForce 6600 GTI and my friends GeForce 6600 both use PCI-E. You don't need a AGP slot for it.

And don't look for AGP. That's outdated. PCI-E is way faster. Rather switch the motherboard if it doesn't have PCI-E (which would be really really really really really odd if it's a new comp).

Google a picture of PCI-E and AGP. maybe you got them confused.