I've read some people/sources say "the timing must match or it won't work". This is not true at least for the Supermicro boards I've tested this on.
etc.. Notice the "12" and "11"
Also the ones that have a "PC2-5300F-555-11-B0" or "PC2-5300-555-11-D0" are also fine.
What happens is what one would expect, the RAM will run at the speed of the slowest chip.