i would hope to god that your existing p/s would support the cathodes. most people can drop cathodes w/o worrying. unless you are using every available power tap in your machine, no need to worry. and also, are you planning on doing any p/s mods to make it clear and drop some lights in there as well?
MacBook 2GHz Intel Core 2 Duo w/ 2GB DDR2 RAM & 120GB SATA 5400RPM HDD
Canon Rebel XTI
Google Cr-48 Beta Laptop