For a start, gaming at 720p, 4GB Ram is totally fine.
GTX750 Ti would be a decent card for 720 gaming. And wouldn't necessarily require you to get a new PSU to support it.
Regards to CPU I'm not sure. First off, upgrade the video card and then test a few games and see where you are at. But for "only" 720p on a PC, I doubt your too far off the mark already if you upgrade your video card.
PREDICTIONS FOR END OF 2015: (Made Jan 1st 2015)
PS4 - 34M - XB1 - 21m - WII U -12M







