system specs can play a role...
Usually motherboard read the SPD value from the stick in DIMM 0 (1st one).
So you have to put the slowest(with the highest timming) in the first one, liek this the motherboard will use those timmings on all the DIMMS(mobo always use the same timmings on all the sticks).
Puttting the fastest oen in the first dimm might therefore cause instability, of course you can always set them manually in bios to make sure!
Asus P4P800DX, P4C 2.6ghz@3.25ghz, 2X512 OCZ PC4000 3-4-4-8, MSI 6800Ultra stock, 2X30gig Raid0