I have Radeon HD 2600 XT and I've searched a lot how to put it in use, and the only thing close to working was a demo of WebGL hashing. The demo wasn't working for me, though in theory it should work on every card with WebGL (OpenGL 2.0) support (Radeon 9500 and up).
My 2600 died a while ago, but it would post 144Mflops in a boinc client cf 1000Mflops for a 4850 card. i.e. slow. Working backwards, if you're getting 250-300 Mhash from a 5850 rated 2+ Tflops, then the HD2600 should be worth around 14mhash. (go forth and conquer the world!)