Super Nintendo emulation has been done very well, and unless you have a slow computer you should get a smooth, eye-pleasing performance. Graphics emulation is almost perfect if you run in 16-bit color. Sound and music emulation is also almost perfect. Mode-7 support, however, is a bit sketchy. The two emulators listed below are the only ones you will ever need.

Zsnes Homepage This emulator has super fast performance and excellent sound emulation.
Snes9x Homepage Slower and less stable than Zsnes, but in most respects just as good or better.