I'd recommend a set of used Rolf Prima Elans. Outstanding wheelset, and one of the very lightest (<1400 gr) alloy production clinchers you can get. I tried a set a few years ago, to see if an upgrade to good-but-on-the-heavy-side wheels really made that much of a difference. I was so impressed, I ended up buying two more sets of Elans for my other bikes. They regularly turn up on eBay for under $400.