I started out on NVidia but quickly moved to AMD.  I would also suggest moving to AMD.  Check out e-bay as even an HD 58XX card will crank out more calculations.  Also start looking to ASICs as given the current jumps in difficulty, we're in for a wild ride as these boxes come online.  The good news is that you can repurpose your GPU miners to something like Litecoin.  No ASIC miners there (yet)