If your machine is shutting down due to heat it's too hot. Remeber when you see 80C in the BIOS after a re-boot it's already cooling down.
Intel CPU's are rated somewhere around 100C and Nvidia graphics cards are rated somewhere over that but that doesn't mean you won't experience issues before then.
I don't like to run any of my components over 60C under full load. My E6700 runs about 45C under load and it's overclocked, my 8800 GTS runs about 50C and my motherboard northbridge runs about 45C.
Have you taken the side panal off the case to make sure all of your CPU, GPU and case fans are spinning up properly? Have you entered the BIOS to set your case fans (and motherboard fan if applicable) to 100% and set up a CPU thermal profile? Have you installed a third party utility like Riva Tuner to control your GPU fan speed?
Most likely your CPU heatsink/fan is not properly seated. Go to Radio Shack, buy some Arctic Silver 5, and re-seat/re-mount the heatsink/fan. You might want to consider an aftermarket heatsink/fan. I'm using an Arctic Freezer 7 Pro (~$25) and it dropped my CPU temps 10-15C vs the stock Intel unit.
Since you're experiencing these problems make sure that, at least for now, you have the thermal managment options enabled in your BIOS (C1E enhanced halt state, TM1, TM2, etc... It sounds like you do or it wouldn't have shut down). People say you should disable these if overclocking but I'm OC'd from 2.66 to 3.2 Ghz and still have them all enabled without issue.
Post here or PM me if you need more help.