^^ Be aware that NVIDIA is not a manufacturer, so the "identical card" from two different companies may perform differently. Be sure to research thoroughly.
Quick question for the OP... are you only using your machine with SONAR? Reason I ask is that SONAR is not graphics intensive, so many cards are fine. 3D and truckloads of onboard memory are for gaming needs. The other aspect is how many monitors are you intending to use, as the ports available will determine choice (i.e., monitor(s) you have now need to plug into it).