XFX is indeed no good, eVGA seems to be the favourite around here, I've installed a few recently and have recieved no complaints regarding them so they must be doing something right.
BFG is also pretty solid, and a lifetime warranty is pretty foolproof.
If ATi becomes your option, the x800XL is what you will want, a bit more pricey than the 6600GT but it's a superb card for its price. I'm not sure if you would be able to find one on an AGP interface though.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w