You don't mention what exactly you want to light but assuming its a PowerMac or Mac Pro the ATA cables have no power to speak of. You can use them to rig an activity LED, but I forget whether that mod is drawing power from the bus.
Most desktop lighting is done with Cold Cathode Fluorescent Lights (CCFLs). These are not unlike the backlights in LCDs. They require an inverter to supply them with their power. The inverter is run from a molex connector (You'll find these powering your optical and hard drives). If you don't have spares, you can get Y-adaptors to give you extra connectors. The inverter boxes can usually power multiple tubes.
If you have an iMac, then you may have only the one molex for the hard drive, but the towers usually have at least a couple.
Search eBay for "CCFL" then pick the computing category, they are used for many other things too.