Yeah from what I understand if you want to control different sets of lights you'll need multiple drivers, one for each set. Like Shawn mentioned you have to know what drivers you're using and what they can or cannot handle. Depending on the drivers though, like if your're using BuckPuck drivers...those require a seperate power supply and you can run one larger power supply to multiple drivers. If you want dimming capabilities you have to make sure you're using dimmable drivers that are designed for that.
Al, I agree 2 3watt Cree's would be more than bright enough to light up your tank. I'd maybe go with 4 myself and just dim them to how I like them. It might give you a little better spread and less spot light effect. But it's personal choice. I plan on running 6 total for my tank...3 cover each side...dimmible though so I can tweak the intensity rather easily.