I gotcha. Understood. Well, for DiRT you probably want an nVidia card then, because the newer AMD/ATI cards are going to be expensive, and the older ones don't do well there.
See what you can cobble together, and then if you need something cheap but w/ decent DiRT credits, look for a GTX 460, 470 (it will put out serious heat, but really good credits as well) and GTX 560 Ti, 560 Ti 448 core, or GTX 570. That is in order of cheapest (least credits) to most expensive (best credits).

Hope that helps a little and let us know what you find to use! Someone on the team may have an older nVidia GPU to sell you for a good deal too - never know, it happens from time to time when someone wants to upgrade.