ok just to clarify for you, memtest is testing your memory not your CPU, so you'll wanna run a program like Prime 95 to stress your CPU.
Have you been bumping your memory at the same time? or is it set to auto?
keep an eye on that as well...
also remember to lock your PCI and PCI-e to 33 and 100 MHz respectively.
finally if your OC fails and you are still with reasonable temerature ranges (under full prime 95 load) then up your Vcore a notch. then try again.
Your MoBo will be the real OC bottleneck here
Depending on your cooling setup and chip type, you could potentially hit 4 GHz or more with that chip! probly 3.7 ish with that MoBo YAYY!
Feel free to ask more questions!
Edit: just outta curiosity, how are you using DDR2 with that setup? that seems a little confusing to me.