6950 in PCIe 1.1

A 6950 will fit into the x16 slot, but it will be seriously restricted
yes, people like Compaq and Dell have been known to reassign psu plug wires on the 20+4pin to 'discourage' you from using replacement parts that they haven't supplied,
I'd say a lower tier card would be a better solution, nvid 450/ati5770/6770 maybe, save the 6950 plan for a new build  
Moto

I suggest going with the above advice. Wait for a new build until Bulldozer and Sandy Bridge-E/Ivy Bridge. Even if you don't want them they should lower the prices of the previous gen stuff to much higher price/performance ratios.