I would not buy a PSU just yet.
The hdd's are unimportant. You don't need them to boot the machine. Disconnect them if you like. The service pack they are referring to is probably WinXP SP2; Vista or Win7 should have no trouble detecting the disks. If they don't, set the disks to IDE instead of AHCI in your bios.
But first you have to get the computer to boot
Try with only a single stick of memory if you have two of them. Try without memory, see if it still beeps. Take out the graphics card and put it back in. Make sure all the power connections to your motherboard are plugged in, I sometimes forgot the 4 pin connector on some motherboard and something similar happened.