Best video card for Mac Pro 2 x 2.8 Quad Core

I would like some advice on the best video card to use with my Mac Pro 2 x 2.8 Quad core machine- 6GB RAM (2008). I currently have the ATI Radeon HD 2600 256MB. I mainly use it for video editing and photoshop work- I don't play games. I have Final Cut studio 2, and Photoshop CS1. I have a dual monitor DVI setup. I mainly deal with Canon 5D Mark 2 footage, and Sony EX1 footage. Recently I have been getting messages that the card can't deal with the amount of requested render in FCP. I am running 10.5.8, but can update to 10.6 if that will make a difference.
Thanks!