First off, they all make the same base card, but may adjust the clock timings slightly diffrently then the reference model, so make sure you check around before buying. The other major diffrence is the warrentee.
^ agreed, EVGA has the greatest support and their gpu's have a lifetime warranty with a 90 day step-up, so if a better card comes out in 90 days you can trade in your GTX 280 and pay the difference for the better/newer card instead of full price. EVGA FTW!
XFX or EVGA - good quality and acceptable support plus heaps of models with factory OC. The only thing I dont really like is the factory cooling solution, but I have always replaced with high end liquid cooling and never had any problems.