Ostensibly nothing, though clock speeds may vary. However, it is important to note that it's up to the OEM to pick the components (VRAM, VRMs, PCB, etc) they use for the card, and can sometimes result in a better or worse product than its competitors (in reliability and potential, not speed, excepting cases where an OEM chooses to, say, slap on an extra gig of RAM onto a certain card).

OEMs also have some control over the BIOS used on the card. This can have a few effects and usually done to tweak clock speeds. I know for my flavor of 5850, the voltage is locked to every OCing utility except Sapphire's own Trixx. Your mileage may vary.

There's also the connectivity of the card. It's not unusual to find display out in a different config than dual DVI and S-Video nowadays. Various manufacturers opt to put varying amounts of HDMI and [Mini-]DisplayPort on their cards, so you'll want to check that out and see which config suits you best.

I'd do some more hunting and figure out if there are any quirks or advantages about those specific cards brought on by the OEM's component choice and other decisions if I were you.