eVGA seems to be nVIDIA's best friend at the moment, Leadtek puts some huge HSF's on so you can do a fair amount on overclocking. I would say eVGA myself though.
If its PCIe I reccommend Chaintechs 6600GT(dont get the AGP, there are better models). But you can't ever go wrong with eVGA video cards. Ive never used Leadtek but me likes the HUGE copper heatsinks.