Buy OEM everything you can; generally a motherboard package will have most of what you need; generally 2-6 SATA cables, an IDE cable, and a Floppy cable. You have the back-plate for the connections, some include a 1 or 2 brackets for extra USB ports, a firewire bracket, some include a COM1 (serial) bracket, a few have a Parallel (printer) bracket.
Generally, budget boards come with 1 or 2 SATA cables and possibly 1 USB bracket.
One last thing; I would recommend an EPS12V power supply. AMS (American Media Systems) has pretty good power supplies; the least expensive in their class ($56 for 450watt, $90 for 550watt). They're well rated.