Hey guys. Just a quick question. Does Zbrush have problems with RAM mismatching. At the moment I have 4gb of mushkin 667MHZ and After I added 4gb of kingston 800MHZ I start getting bluescreens sayin PAGE_FAULT_IN_NONPAGED_AREA. So I sent back the kingston Ram thinking it was broken. got some more and I get the same problems.
my specs
AMD Phenom quadcore
nvidia 8800 gt 1gb
You can run into all sorts of problems with mismatched RAM, and not just with ZBrush. This is especially true if the different sets of RAM require have different voltage or timing requirements.
My bad. the mushkin Ram is also 8000 mhz. both are 1.8v and have cl5 timings so the only difference I can see is the brand name.
800 not 8000 Mhz. 
Have you tried running a RAM diagnostic program?
I ran Memtest ont the new memory sticks all last night and both came back with no errors. so all 8gb are fine. I guess its just zbrush not liking different brands of ram