i've tested many (if not every) manufacturer out of ATi. Good card manufacturerrs out of ATi chipsets are:
ATI - themselves (most recommended)
Asus
MSI and GigaByte have recently started, just to to keep themselves in competition, though it is believed that they might make good cards.
Powercolor - hmm, OK, but their 9700 cards were faster than the Original ATI Built, havent tested the 9800/X800 chips though
Sapphire - its on your Luck, if u get a better piece out of their production, else keep changing until you get stisfied fully.
Hercules - they were very nice as well, unfortunatel given up making graphic cards.
Tyan/FIS/Connect3D are just helping ATi to spread their video chipset into more and more PC's