The cool thing about the 6950's is some of the 2gb versions can be flashed to a 6970 ($100 more) with a quick bios switch. For obvious reasons vendors have been slowly "fixing" this, but some cards out there are still able to do it.
This card can normally always be flashed, when you can find it in stock. Key term is "flex 6950" by sapphire.
Aside from that the differences really mean nothing, the only difference between "Sapphire" and "HIS" is the cooler/heatsink that goes on top of the card, Radeon sends out the reference card to these different vendors and they develop coolers for it and sell them as a "Sapphire Radeon 6950" or "HIS Radeon 6950"
"XFX Radeon 6950" - XFX cards have a lifetime warranty, if you go that route just make sure and register it properly when you get the card.
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