I've been video starved with my MP 4,1's for some time now (stock ATI cards) and I decided to experiment with Quadro options. Of course, I bought one of the expensive 4000 units from Apple, but I also found what appeared to be a "too good to be true" bargain on eBay. The seller claimed a Quadro 4800 w 1.8GB of RAM with full Mac Pro support for $333. They also offered a reasonable return policy and had a 100% customer rating, so I bit. They are listed as used, but it turns out that they are generic OEM, white box cards. The card arrived, and aside from not having a fancy Lexan logo sticker on the fan shield, it was really a 4800 and it included the Apple power cable.
I've now tested two of these on a pair of MP 4,1s with total success in running OpenCL and CUDA (CS6) apps.
If you're looking for a great performance boost that won't break the bank, check out this link: