The only difference is what you get with them.
They performs exactely the same.
Heres a small run down of the major differences between the different makes.
(Basic SW/HD Bundle = Drivers, DVD Player, S-Video Cable, Power Cable, DSUB-DVI converter)
ATI Brand - Basic SW / HD bundle
Sapphire - Basic SW / HD bundle
Sapphire "Silent" - Same as above, just with Zalman HS unit instead of HSF Combo.
Hercules - Basic SW / HD bundle, Memory Heatsinks, Better VPU HSF, 1 Game
Gigabyte - Basic SW / HD bundle, "Front memory Heatsinks, 4 Games.
Tyan - Basic SW / HD bundle, Hardware temperature monitoring, memory heatsinks.
Connect 3D - Basic SW / HD bundle
Creative Labs -Basic SW / HD bundle, memory Heatsinks, 1 game.
This is going off my memory, so there might be mistakes.
If ive missed anything, then someone shout up. or if ive added something that isnt included then shout up as well.
I have the sapphire one, and it performs great. It all comes down to:
Your budget
Your Needs for any extras.
Hope this helps.