Currently you can get
-520m, which can't do much at all;
-540m or ati 5650 (those 2 are identical in perfmce);
-285m (big perf difference from above 2 and 555m would be exactly in between);
-460m, 485m with fair difference in perf;
-460m sli, the best available but the 485m sli would kill it
So the next best thing to the 555m is the 540m or 5650 and the next step up is 285m.