If you clear the bios and rip off all of the stickers you would never be able to sort them back out. The cards that you listed are all reference cards meaning that they are all clocked to default ATI values and have identical parts...it is quite possible that some of them came from the same assembly line.
So to add to Fleakiller's comment, you have to look for who offers better perks. Can you get a free game out of it? A rebate? Can you get a longer warranty? Is there good customer service with that company? You get the picture.
Newegg has the Visiontek with a free copy of Grid AND a rebate...now that is a swell deal.
there are other differences as well ( not with the card) some company give you alot of cool things like a free game/software and other cool things too. the best example of this is sapphire as they give quite alot inc 3dmark06 advanced full edition oh and on the better version they change the cooler as well...I haven't seen a HD4870 with a better cooler yet apart from the powercolour 1Gb version