Awesome. Well, I was told by an electrician that in order to find out how many amps your PSU has available is. For Multi Rail Systems, You take how many watts are available on your 12V rails( In your case 313) and divide by however many volts you have (12).
313W / 12V = 26A
So you has 26 Amps to work with.
From what Ive read you only need 350W PSU and with a system containing all the other essential parts for operation, 20Amps. I recommend a GT 630 for a increase in performance, Not having to upgrade PSU and not breaking the bank.
Mind you I had my method confirmed by users of this site when I was asking questions about upgrading my GPU a few weeks ago.
I may even be being a bit conservative.
My original thread- http://www.tomshardware.com/forum/forum2.php?config=tomshardwareus.inc&cat=33&post=380103&page=1&p=1&sondage=0&owntopic=1&trash=0&trash_post=0&print=0&numreponse=0"e_only=0&new=0&nojs=0
PS - Dont let the dust bunnies take over your FANS! =D