Yes, it would definitely be worth buying an i3. See here, an i3 absolutely destroys a Pentium in Crysis 3, which is very heavily CPU dependant: http://www.tomshardware.co.uk/ivy-bridge-wolfdale-yorkfield-comparison,review-32682-10.html
I actually own a Coolermaster Elite 500. It's not a great power supply, but it's good enough for an i5 + a Geforce 660. It's not even 80+ certified, that's how basic it is. Despite that it's powering my Core 2 Quad (95 Watts) and Geforce 460 (170 Watts) based desktop quite happily, and they have higher power requirements than an i5 (77 Watts) or a Geforce 660 (150 Watts).