The quick answer to this would be to exchange the 5870 for a Nvidia-based midrange card - sl is developed on Nvidia hardware.
The more complicated answer is that in the drivers of AMD/ATI hardware there is a problem with memory allocating within opengl, the way sl do. I dont remember exactly what, but you could google it. It has bugged me the last 5 years and I do have a 5870 myself, which in performance is miles ahead of the forementioned GTS250.