From the specs I can find online, the power supply is only 220 watts. Its nowhere near what you need to power the GTX 580. You can find those specs pretty easily.
The dual core pentium processor would be ok if you were running AAA titles released from say, 2005-2011 or so. It's just not got the oomph to power the GPU adequately imho.
Now, as far as the GTX 580 not fitting in this case, that's an issue. I believe the motherboard is probably a BTX type or possibly even custom, so a simple case swap is not so simple either.
The GTX 580 is still a decent card, but it'd cost too much money to support it properly. It's a bad price to performance ratio all around.