First of all, don't buy used 6950s for $150 each when there are plenty of deals for NEW 7950s that are $220.
Would agree with this. 5xxx series or 7xxxx series will be much more efficient.
If you're using GPU's to mine, I'd suggest mining on middlecoin or something similar (i.e. Multipool). At least you'd earn much more BTC than you would mining straight BTC.