Is it worth the extra $60-70 to get that extra core? How much difference does it make for gaming? Btw, I will be overclocking with an Hyper 212+(2 fans). I realize it will make a pretty noticeable difference in rendering, but is it worth the extra money? I'm on an extremely tight budget, but I'll do it if it helps a lot.
I also realize I have the chance of unlocking the 4th core on the 720. What's the chances of me getting a clean core?
The PC will be mostly for gaming and minor Photoshop, After Effects, and Cinema 4D.
For a gaming build, the Phenom II X3 720 will perform very well. Tom's did an article a while back and it showed there is little benefit in more than three cores. As you stated, the rendering will be impacted by the lack of cores.
I would go with the X3 720 for your build to keep your budget down... IMO