If you're gonna upgrade your wheelset...make sure it's a viable upgrade. If snappy acceleration is what you're seeking, research your current rim weight, spoke weight, brass vs. alu nipples, tire and tube weight. Rotating weight feels like an exponential increase in drag when pedaling.

Say your wheels weigh 2300g but you're running XT hubs...you won't notice much of a change in a 1900g wheelset b/c the weight saved will be in your hubs where the fulcrum rests.

Definitely try to score good quality hubs, but try to concentrate the weight loss in the rims, tires, and tubes. Don't get stupid-light though. I've settled into a 2200g wheelset (IIRC) b/c it's tough enough. I could possibly get into something around 1700g for a similar price, but I do belive they'd live a short and battered life.