2GB cards help you with higher resolutions and if you are going to do multiple monitor setups. The 2GB it is also for graphic intensive games such as Battlefield 3. You can't go wrong with 2GB of VRAM, especially for current generation games.
You may also consider a 7850 OC edition, it has better technology, lower power consumption and barely any price difference between the 6950 and 7850.
Well, honestly all PCI-E slots are backwards compatible. It will adjust accordingly. And I wouldn't post the benchmarks if that weren't true. There are a number of people on this board who uses PCI-E 2 cards on PCI-E 1 slots.
Maybe for the rare occassion of 2.1 to 1.0 sometimes doesn't work. But, 3.0, 2.0, and 1.0 are compatible.
with the hd 6950 what power connecter do i need for it
AMD Radeon™ HD 6950 Graphics
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended AMD Radeon™ HD 6970 Graphics
550 Watt or greater power supply with one 150W 8-pin PCI Express® power connector and one 75W 6-pin PCI Express® power connector recommended