I suggest to just look at the price + performance deal and see if it would be worth your penny.
Basically, they're all using the same chip inside. The difference only would be the way they present the product with custom clock/shader/mem config, cooling solution, warranty, packaging, aesthetics, etc.
The best way to see which one is the best for your needs is to check out reviews.
One difference is that some brands sort their cards for overclocking and offer more overclockable cards for more money. It's better to go with a brand that doesn't do this if you want a baseline card because you might get one that overclocks well.
If you buy a baseline card from a company that sorts it will most likely not overclock well otherwise they would have sold it at a premium.
I kinda doubt they even do that. Most factory OCs are well below the speeds the average card can reach so they probably just OC random cards and maybe toss back the ones that can't take it but I'll bet that's rare. I could be wrong though.
As a non OCer, the most important thing for me is having a card that regulates it's fan and is quiet.
My current 5870 is extremely quiet and I can't even tell when it's under load, but the 9800GT I replaced it with was extremely noisy under load, as the fan went from very low to 100% with no inbetween fan settings.
It was talking about Windows 7 64-bit editions and driver issues with ATI HD5750 cards. People were getting game crashes, some people would get BSOD and so forth.
Stuff like: "Driver support and updates are questionable. I am still on occasion getting a system that crashes to a solid or stripped screen and tech support has not come up with a valid resolution."
I had similar issues when I first installed my Diamond 5870, although the drivers that came with the card never worked, I used the ones on AMD's site.
The issue went away once I OC'ed the card. Apparently if you OC it, whenever the card isn't using 3D graphics, the memory clocks drop much lower. I'm guessing that allowed the memory to stay a bit cooler and it never did that crash again.