Depends for what. For all around, training, commuting, etc, I'd say 105/Ultegra with Mavic open pro's is a great combo. It is a very standard wheelset, so replacement spokes, rims, hub components, etc are easy to come by. Just a note, if you buy from performance or nashbar, have an LBS check the tension or possibly rebuild the wheels. The build quality from these places is hit and miss.

Mike Garcia (oddsandendos.com) makes great handbuilt wheelsets. They are a bit more pricy though. Still well under $500 though.