Re: how much gb
If you are in US then for around 100$ to 150$ .. you can buy a computer (desktop, excluding monitor) with reasonable spec
RAM 256, CPU 1+ GHz, 20GB HDD
Thats minimum you would like to have for the "now a days" technology.
Depending upon your budget ... if you can spend some then I'd rather suggest to buy a new computer .... if I'd be you .. I'll look for a new computer (excluding all celeron models).
Good luck with ur computer hunt.