I would love to help you out; unfortunately I havent had my hands into a similar rig yet I am ashamed to say
I may get an oppurtunity to play around with one in the next couple of weeks at the college computer rec center where I vilounteer, but right now we only have two systems similar to yours and there are loads of dudes that are "playing" with them.
so I have avoided being associated with these two machines / if we blow some parts; those who have been "playing" must pay and right now the "boyz" are being very wreckless with the clocks on these two machines / so I have folded my cards for the time being
maybe someone else will chime in
I can tell you this >>>>> you need to find the performance voltage setting for your ram (highest voltage manufacturer lists) and set it there
also just begin raising the cpu clock until orthos fails >>>> then back off
there are versions of Prime and orthos that are meant to be run by quads >>>> it will stress all four cores
let us know how you progress and sorry for the lack of detailed help
its in the works though
