I would invest in things that could be transferred to the next bike. I am a big advocate of wheels that are one step up from stock being a big bang-for-the-buck. Neuvations were good for this, but some Titans or Corsa lites look good as well. Also, Tires are probably a great investment regardless. Buy the best you can get, and save the old ones for a trainer wheel or a mishap later.

I always say that the best performance improvement I ever saw on my bike was to upgrade whatever my Allez had for stock brake-pads to Coolstops. I have spent hundreds to upgrade my drivetrain and shifting, and the difference is marginal, but that $35 on those pads increased my braking ability more than I can measure.