Say what you want about Microsoft but their mice are the best (next to logitech) go for either the intellimouse with scroller ball or if youve got more dosh the lovely laser one (comes in cordless too)
also avoid ergonomic keyboards as they are suck.
Like Red_Baron says if the PC is for gaming get good sound/graphics, if youre gonna use it for doing essays and surfing the net just get cheap (or onboard) sound/graphics. Soundblaster Live is very good though, and dont forget that no matter what you spend on a soundcard it can be wasted by having crap speakers.
motherboard, most should have AGP and USB anyway but make sure it does as most likely you will need these ports at some point.
i was away to say "thats all the general answers unless you state specifically what you need" then noticed you only asked where to get a pc cheap, so probably know all this anyway and we seem like tubes, excellent
