i guess start out at normal speed and see at what point it fails
are you just trying to run the memory faster or are you o/c ing the cpu
if its the former perhaps running the ram at the normal ram speed for your cpu would allow tighter timings
(you need to benchmark to see if its better)
if its the latter and your not changing any sort of cpu/ram divider 1866 is nearly 100% overclock no wonder its failing
ps: found this on wiki (no idea if its true or out of date)
"in early October 2008, reports surfaced that it will not be possible to use "performance" DDR3 DIMMs that require voltages higher than 1.65v, because the integrated memory controller within the Core i7 will be damaged"