It's been awhile but the best bicycle equipment change that I ever made was switching my Klein's crankset from a 53/39 to a 50/34 compact. That was in the octalink BB days so I bought a crankset that had the same interface. I installed the new crankset, lowered the front derailleur, and resized the chain. The whole process took under an hour and only thing I had to buy was the crankset.

Essentially I traded a couple of gear ratios that I never used for a couple of hill climb ratios that I needed.