I've got two different memory manufacturer in my case. One is a OCZ platium edition with 4-5-4-15 and the other is a Kingston ValueRam running at 5-5-5-18. Does it matter that these run differently even as though both run @ 800MHz?!
Well my friend I would not reccomend that to any one. If it does work on your M/BD they will usually default to the slower timed stick of ram as opposed to OC'n the slower stick to match the faster timings.
More importantly probably thou is using different sticks like that can give you stability issues resulting from corruption of your OP/SYS files.
I would reccomend that you use a matched set of ram assuming your M/BD supports dual channel mode ( as most do now adays ) or not just for the stabilty issues involved.