Just so you know, a 6600 GT won't maximize your system. Close, but not quite. You're CPU and RAM can keep up with a faster card. I'd recommend a Radeon X800 XL or 6800 GT or equivalent.
When it comes to video card brands, generally I just look at the reviews and newegg and compare all the same chipsets to make sure it's not some half-assed card with the memory speed 60% of Ati/Nvidia's stock spec.
I have an BFG 6800 Ultra and it's been solid.
I had a sapphire 9800 Pro and it was good, but it was a bit slower than other cards. I got 5-10% lower scores than people with nearly the same exact setup as me but with different brand video. And I had all the latest drivers and chipset stuff so it wasn't software.