Krusty, the reason for the big price diff in a 6800GT PCIe and a 6800GT AGP is that the 6800 was originally designed for AGP. It runs natively in AGP mode, and has to use a "translator" to work as a PCIe card. The 6600 series cards, however, were designed to run natively in PCIe. The thing that slows them down so much is the lack of pipes compared to the 6800, and the much less complex processor. The 6800GT PCIe card is faster than two 6600GT cards in SLI mode. At least in most tests.
Second, that $99 link is for a standard 6600. Far inferior to the 6600GT even. Not something you want to waste money on for gaming, when for $50 more you can buy the GT model. Its an acceptable entry level card for current games. Stay far far away from the 6200s though.