Simply put: More light (more lumens, not necessarily more watts) and a big root system will give you bushier and higher yielding plants.

Upgrading the lights is a good idea, but don't replace them. Just keep adding more lights (Y socket splitters and clamp light fixtures from home depot or walmart). You should have done what you are planning at least a couple weeks ago. 4 x 42w I'd say is the minimum you should have for one plant. This guy has a pretty nice setup with cfls:
http://boards.cannabis.com/grow-log/...ars-green.html