OLD outdated chart, but show back then that only about a 10 performance between SLOWEST and fastest ram: http://www.tomshardware.com/reviews/ram-speed-tests,1807-8.html
Same applies to day with faster ram. You will see almost NO real-life performance gain between 1333 and 2000 (with the exception of a few select apps)
Not sure about AMD, but with Intel SB CPUs 1600 was optimium.
PS as speed decreases, compatability general increases.