RE: Optiplex 990 supported memory speed
JEDEC DDR3 are 7-7-7-20 for DDR3-1066 (PC3 8500) and 8-8-8-24 for DDR3-1333 (PC3 10,600)
Timing 9-9-9-24 DDR3 1600 (PC3 12800) is backwards compatible to 1066 providing it matches the 7 - 7 - 7 - 20 timing.
The speed that it runs has nothing to do with the dell model and everything to do with the frontside bus of the CPU. In newer models the memory hub is INSIDE the cpu.
In other words you don't get faster ram with a slow cpu. Request denied.