I should not think there will be a problem with your memory configuration given your current circumstances.
Your memory is rated to operate at 800MHz, with acceptable latency periods, therefore there should not be a problem with that particular part of your computer system (2.1005v rated voltage).
Unfortunately, after a given clock frequency, the Core 2 Duo Architecture has to have a massive increase in system voltage to compensate for these clock frequencies.
Here is an example chart I have created.
You can see that voltage will drastically increase after a certain point.
I therefore suggest that you do indeed increase the CPU Voltage to compensate for the clock frequency. All CPU's are different, you may require a higher voltage than others to ascertain your desired clock frequency.
Also, I really doubt that the memory is to blame as Orthos does not stress the memory a huge amount, unless it's repeatedly paged. Orthos mostly just does 32 bit random allocation locations, which are not particularly stressful.
You can of course increase the voltage of the memory if you wish to ascertain tighter timings, or just increase overall stability beyond what is required.