My mobois A00 0P611C (Chipset nForce 680i SLI SPP Rev. A2), and the QX6850 can OC and stable at 4GHz... If DELL's BIOS can support higher than 4GHz, i'll try for 4.33GHz...
I did try 4.3GHz before, no luck, no post...!!!
Actrually i received H2C unit before the DELL 1000W PSU. so connected H2C unit with 750W PSU, then OC QX6850 to 4GHz (but only use one XFX GTX 260 XXX edition)
i did OC via BIOS, under performance, select 333x12, 4GHz (default is x9), if i remember correctly, the voltage is 1.58V at x12. I tested 3dmark 06 for one hour, no problems.
u do via BIOS, set multipier to x12 first, then boot into windows, do some stress tests. If passes, great.
BSOD, then re-enter BIOS, decrease multipier by one each time till it's stable.
I just got lucky, my QX6850 is quite a Monster...!