750w PSU (I think it could be my PSU as its a old, no name brand)
, but its still better to upgrade, your PSU might not be able to keep up in the next few years
Go for any PSU (At least 550W) from any of these good brands-
But before you do that, check-
Is your drivers the newest one (12.10 minimum?)
Have you uninstalled your Nvidia drivers (Shouldn't be the problem but just incase)
Are your cables connected properly?