http://www.giant-bicycles.com/en-us/....1/9041/48915/ This guy?
Sooo...I don't want to tell you what to do, but I'd upgrade that fork before I changed the wheels. You might take a bunch of rotating weight off with the wheels, but a solid fork will do absolute wonders for your ride. Way more important, IMO. Cobalts aren't cheap...for that same money, you could probably put a Float 29 on there. Your bike has a 15QR, right? I think Fox makes a Float with a 15QR, so you wouldn't need a front wheel yet. Looks like you'd need a 1-1/8 steerer, but it's hard to tell from the picture.