really the bare minimum you need for a pc is a motherboard and cpu.
you could reuse your current pcs case, ram, psu, hard drive(s), optical drive(s), keyboard, mouse, and monitor. unless an upgrade is required (like the change in ddr to ddr2)
on a side note...if you just want to get the cheapest there is, theres not much use. i somewhat went that route with my current pc when i built it in october. i'm kicking myself now for not getting nicer stuff since i think i already have to upgrade it since it possibly can't handle a geforce 8800.