i have the XFX 6600GT PCIe, and i like it a lot, especially considering it's usually the cheapest out there. The blue PCB and dual DVI outputs really set it apart from the others. As for overclocking, mine's sitting at 543/1090 right now. I'm not sure how much higher i can go, but i know there were little green artifacts at 552/1120, not sure if that's the upper limit for memory, GPU, or both, though. Anyways, it's real stable, and the dual DVI is a must for LCD owners. The only thing i found lacking was the software bundle, which included (yawn) commandos 3, X2: the threat, and some stupid nascar racing game. Of course, i've had no personal experience with the AGP version, so i don't know how it performs relative to the PCIe, but i know they use the same stuff other than the bridge chip.
Oh, yeah, and if you can find a BFG or gainward, they are great, but I haven't seen any of them available yet, even though they both "launched" their overclocked product lines several months ago. Man, I hate paperlaunches.