Could a 550 watt power supply support multiple hard drives while using a Radeon HD 4850 1GB 256-bit DDR3 PCI Express 2.0 x16 video card?
If it is a nice 550, like a Corsair, then it should be able to handle several drives plus that video card and a nice quad-core. If you want to run hardware RAID, you should even have enough extra power for the card. If you plan to keep adding drives in the future, you might want to go with a 750W just to save yourself from buying it in the future.