Is there something about your current wheels that you don't like? To really notice any improvement that your wheels give you, you'd have to be in full-race situations -- we're talking a few seconds per mile saved. You're not going to suddenly get 2mph faster when out riding with your friends because of new wheels.

If you're just looking at overall reduced weight, then as you've noticed, you have to spend a small fortune to drop a signifigant amount of weight. It's obviously your money and only you know what you can afford, but I personally don't see the point in dropping $600 on light-weight wheels to upgrade a $600 bike.