For an external hard drive, get one with Firewire 800 running at 7500 rpm. I would then transfer all non-essential files from the internal to the external drive and use the internal for making movies. iMovie6 is still a great little program for making movies but I also use Final Cut Express for more complicated procedures.
I also concur with the suggestion of not buying HD camera but stick with mini-DV tape camera. If you can squeeze in a Sony VX2100 camera, you’ll be set for indoor work with minimum of lighting equipment.
