how much money do you want to spend?
I just upgrade my wheels. Zipp 606 with Tune skyline Hubs.
On a recent test I went out on my Kysiriums SL and climbed a hill 5 times and free wheeled back down it. I then changed to the Zipps and performed the same test.
It was not scientific but the Zipps where 3 mph faster on the down hill and 30 seconds faster on the up hill even though I was more tired!