Question : Problem: I have a Phenom II X3 710 and 4GB of 1066 RAM. The third core on the cpu doesn't show and the ram won't run stable @ 1066. How do I fix this?

My system:
Motherboard - Gigabyte GA-MA78GPM-DS2H (rev. 1.x)
RAM - Crucial 2x2GB PC-8500 1.8V CT2KIT25664AA1067
CPU - Phenom II X3 710
OS - Vista Business 32-bit

I just upgraded my system from 2GB of PC-6400 and an Athlon 64 X2 4800 to 4GB of PC-8500 and a Phenom II X3 710. In the device manager all three cores for the Phenom show up but in the task manager it only shows two. How do I get the third one to work? By default the new ram wants to run @ 800 but I was able to set it to the advertised speed of 1066 in the bios. However, when I do that Windows randomly crashes and Memtest finds tons of errors. Memtest @ 800 shows no errors at all. What settings does my ram need to run stable @ 1066?

Update:
I just went on the Gigabyte website and saw that they just released a new bios. I went ahead and installed the new bios but there is still no change. It only shows 2 cores. I haven't messed with the ram on the new bios yet.

Answer : Problem: I have a Phenom II X3 710 and 4GB of 1066 RAM. The third core on the cpu doesn't show and the ram won't run stable @ 1066. How do I fix this?

You installed Vista with only two cores.
Adding more CPU's is not Plug'n'play after the OS is installed.
You will need to reinstall so Vista knows how many cores are there.