MSI are the cheapest (but actual speed is less than other cards)
Gigabyte are stable, but again not exactly amazingly fast
Asus are often external driver incompatible (you can't use the native Forceware)
never used AOpen so dunno...
my suggestion:
Creative or PNY for Geforce
Sapphire or ATi for Radeon
the other makes your really just taking a gamble with the speed and stability; Good example is the fact that my 5200 Creative actually outperforms my mates MSI 5600. (both non-ultra)
As for the 6-Series, no retailers have really officially started selling yet. Even though they should've been on the shelves for 6th June.