I bought a radio frequency wireless keyboard and mouse yesterday at Circuit city for $30. It works really well. If you go wireless my advice is to make sure its radio frequency and not infared. With infared you need to put the receiver somwhere that the keyboard can see it. With radio frequency the receiver can be inside your cabinet.
I also bought a cheap 6 button joystick for $1 that looks like its going to be easy to modify for my mini-mame project.