I'm not an expert on wheels, but I encouage you to put as much thought into your choice of tires. When I upgraded, my pace improvements came 5% from the wheels and then another 5% with my tires. That combined 10% improvement compares to an only 7% pace improvement with the purchase of my Madone!

I ride RaceXLites (purchased new on eBay) with Vittoria Open Corso tires (which I absolutely love!)

Good Luck