Wheels ARE the one best thing you can do to your bike.

Mavic Ksyrium SL or sommat from Zipp. I love my Ksyriums, they are sexy.

A carbon seatpost... oops that doesn't fly with the new Madone seatpost system.... ok at least a great saddle.

Get some good light pedals. Don't get Time, maybe Look Keo Carbon if you want to do yourself some good.

ps. the 12-27 cassette works just peachy on a double, maybe the LBS will swap that for you.