Buying a new computer is for unexperienced. If you are a more experienced computer used, it costs less money to get the parts individually. I don't think you need a new Power Supply or Case, these already full up some $. DVD Drive? Wtf almost everyone has one. Graphics Card? Well maybe, unless you have a better one already. Cooling System? Unless there are more cooling vents, I wouldn't go for this, unless I want a new cooling system...
I think you will already save around 40$ for the stuff I listed, I have no idea how they would cost individually.
Actually there are 2 things a full computer offers: 1. You don't need to replace / insert parts in it, I think. 2. There's usually a discount, but if you want to make small changes it's not recommended.
I run on:
Motherboard: P4GPL-X Asus.
RAM: 2 x DDR1 ports. ; 1.5 GB.
Video Card: DDR2 EN9400GT Nvidia (-Asus).
Processor: I don't even, Pentium IV Single core 3.00 GHz.
DVD Wr- none apparently, dad uses mine because his DVD Writer (/reader) broke.
And other stuff I don't know about, never wanted to know about really.