I've bought from BgMicro several times, so i've been through this before. You will need an inverter (not a 3-4 pin converter) for each cathode. In addition, i highly recommend getting some protective tubing, as the CC's themselves are fragile, and ironically enough are anything but cold. You don't have to have a rocker switch or the like, but it makes it much easier. I personally went for a baybus/fanbus, which allows you to control the intensity of the lamp and speed of the fans from a front access 5 1/4 bay. Not to mention i didn't have to mess up my nice paint job by cutting a rocker hole in the front. The inverters from BG are not protected in any way shape or form, and give you (or your hardware) a nice jolt if you're not careful. Do yourself a favor and get a film canister or other such plastic container to place them in, and mount. All of the mod sites are overpriced for these. If you don't want to mess with piecing your own CC's together, BlackScorpion (AnAndtech member id) used to sell them all the time in the Hard forums, and here for a bit too. I've bought some from him and have nothing but good things to say. I don't remember his pricing exactly, but he was much more reasonable then the online mods sites.
MoFunk, You can run two cathodes from the same switch, but if you're going to be using more then one, save yourself the headache and make/buy a baybus.