If you have the means then I think a wheelset upgrade is the best most noticeable upgrade to your current bike. It's true you may only be marginally faster if at all but you'll notice it more in changes in acceleration, plus it's a great motivator to ride more and harder- especially up the hills. I've been curious about Boyd's as mentioned in another post, I have Dura Ace C24's on one of my bikes and it's an exceptional wheelset in terms of weight, stiffness, and comfort.