My personal favourite is Hyper Athlete. It sometime suffers from slowdown but not every time. Also the sound isn't 100% correct. Tts like a modern Track & Field which is cool.
Not sure if 256Mb of RAM is enough though......
Hope that helps.
Buks
PS Are you downloading CHD's ? Much better to use one of the ROM burners (non-profit making AND you get a full set including all CHD games).