Engine upgrades. It's like a video game. The more you play, the better engine you have. If you still have a mountain bike, take it out and do some really good technical singletrack and dirt hills with it on the weekends. Work on endurance during the week (maintain a little higher cadence than usual all the way home, for example) and don't worry about the bike for a bit.
Really, the human on the bike is almost always the cheapest to upgrade, but you have to do it over time. It also happens to be the weakest link most of the time. For instance, there's no sense in me worrying about the difference between a 7 pound madone and a 35 pound schwinn varsity when I'm still more than 40 pounds overweight. There's no sense in worrying about infinitesemal inefficiencies in bearings and drivertains when my lungs, heart, and legs are holding me back. There's no sense in going for aerodynamic wheels, bottle cages, or handlebars when my flabby belly and thick wasteline are acting like a barn door.
The way I figure it, work on the engine first.