A good idea may be to buy a piece of junk p3600 or something like that really cheap at a 'warehouse' type place. Should be able to get one for near 200 dollars now. Then, just add an okay vid card (tnt2 ultra with 32mbs) for 30 bucks. You are set to go.
Then when you can afford it you can upgrade to the good stuff, without buying a case, power supply, harddrive, floppy, cdrom, keyboard, mouse and all that other crap that has stayed the same for 10 years.
Just dont buy a proprietary system, ie something with a name on it.
Get a microtek or some piece of crap like that.