It doesn't take much and it can be very inexpensive! At the very least simple white up lights will do the trick, but if you are feeling a little more in the mood to spice your reception up a bit, pick one of your accent colors and run with it...but be careful and use sparingly when using green or blue up-lighting--it makes people look sick and blue lighting suppresses the appetite! Always make sure to hire a professional for your lighting needs!
Example: Here are before and after pictures from one of my weddings last year--what a difference the lighting makes! The simple up-lighting in this picture ran about $300 for the entire room!
BEFORE

AFTER
