I’ve had a heatsink on mine since long before I started running RetroPie but in a rather closed case with just the heatsink it could easily go above 70 degrees celsius. Now I’m using a wide open case with heatsinks and fan and it runs about 25 degrees cooler now.
My overclocking is:
Important for overclocking is a good powersource (700mA is not enough, I’ve tried).
Also I believe cranking up the core clock may be what have the biggest effect. Overclocking the gpu probably have very little or no effect. I went from 250 to 275 just to make it half the core clock. It most likely makes no difference.
I have three Raspberrys and all three are stable at cpu=1000, core=500, sdram=500 and overvolting=6 if used with a good powersource. That’s without active cooling. Only the one I use for RetroPie has been taken it bit higher.
Here’s a script that you can use to monitor the temperature:
Btw, here’s my overclocking settings from my Raspberry that runs Raspbmc:
I wouldn’t be surprised if your Raspberry can run with those settings too if you have a good enough powersource (1A or more).