Some comparison shots of changing the grass density:
iMinGrassSize=80(default), Avg. FPS=17
iMinGrassSize=120, Avg. FPS=20
iMinGrassSize=150, Avg. FPS=23
iMinGrassSize=200, Avg. FPS=26
These are on a 7800 GTX 256 OCed at 1280x1024, HDR, 8xAF. The FPS values are approximate as I didn't use fraps to do a benchmark run.
A value of around 130 seems to be a good compromise between looks and performance for me.