OK, well we'll all put our heads together and find you something. I wouldn't got any lower than a GTX 460, and preferably a 465 or 470; if you go as low as the 400 series - this is for high output on DiRT crunching I mean. The 500 series is best, but that is certainly going to be more costly and we're looking for inexpensive here...

On the ATI side of things, you could get by with a 5870 or so, but the problem is they weren't really good at DiRT output until the 7900 series. It has to do with the architectural changes to the GPU style, and how it uses GPU compute commands.
Keep us in the loop on what you find!