Aerobars first, then a aero helmet will probably be the most cost effective solution for a bit of extra speed. You won't need a new bike at all. Then just work on tweaking your position. Make sure your tires are also pumped up to an adequate level, maybe even 5-10 psi higher than normal (but not over 120 psi for most people).

Here's a good site for tweaking your position:

http://www.bikesportmichigan.com/bikes/karma.shtml


But most importantly, you'll just have to work on improving your own cycling ability through training.