The thing is, medium budget for workstation = able to buy 2 HD5870, which is why it matters, it's still smaller than a breadbox... err... cheaper than car, but it's not a medium gaming budget by a long shot.
However as an HD4870 stock out of the box is about equivalent to an entry level FireGL and Quadro despite their optimized drivers, then you can focus on something above an HD4770 or above a GTX265 and do well. The HD5800 series will give you the best of both worlds right now, but you may have initial viewport issues depending on how well the initial drivers were written for general OGL support.
If you were using something DX based like 3DSMax it would be a little more predictable and nearly similar to high end workstation performance. OGL apps still benefit alot from the workstation drivers, so even a moded HD4870 or GTX/S would be a good option if you're comfortable with modding.