Which CUDA card for a Mac Pro?

I've recently started using Premiere Pro CS6. My current setup does not make use of the Mercury playback engine.

Can anyone recommend an appropriate CUDA graphics card that would allow this?

Ideally I don't want to spend more than £300.

Here is a breakdown of my setup:

Mac Pro 4,1:

2 x 2.26GHz Quad-Core Intel Xeon (2 processors, 8 cores)

32GB RAM (DDR3 1066Mhz)

ATI Radeon HD 4870 (512MB VRAM)

Black Magic Decklink HD Extreme 3D+

x4 3TB 7200rpm Internal hard drives (i.e. 1 root drive and x3 for footage)

OS X Version 10.8 Mountain Lion

Thanks.