This website here will give you a lot of great insight on the cores, choosing what's best for you AND even give you tips on enabling the Mercury Playback Engine GPU Acceleration on Non-Supported cards!
http://www.studio1productions.com/Articles/PremiereCS5.htm
Also Adobe Recommends the following:
-GeForce GTX 285 (Windows and Mac OS)
-GeForce GTX 470 (Windows)
-GeForce GTX 570 (Windows)
-GeForce GTX 580 (Windows)
-NVIDIA® Tesla C2075 card (Windows) when paired with a Quadro card as part of an NVIDIA Maximus™ configuration
-Quadro FX 3700M (Windows)
-Quadro FX 3800 (Windows)
-Quadro FX 3800M (Windows)
-Quadro FX 4800 (Windows and Mac OS)
-Quadro FX 5800 (Windows)
-Quadro 2000 (Windows)
-Quadro 2000D (Windows)
-Quadro 2000M (Windows)
-Quadro 3000M (Windows)
-Quadro 4000 (Windows and Mac OS)
-Quadro 4000M (Windows)
-Quadro 5000 (Windows)
-Quadro 5000M (Windows)
-Quadro 5010M (Windows)
-Quadro 6000 (Windows)
-Quadro CX (Windows)
-Tesla C2075** (Windows)