The default RAM frequency for Phenom II CPUs is 1333 MHz. The default RAM frequency for Bulldozer/FX CPUs is 1866 MHz. Most mobo makers have a QVL list of tested RAM. Many RAM suppliers also have a RAM selector where you can find RAM approved for specific mobos or CPUs or brands of CPUs. If all else fails you can e-mail Tech Support for the mobo and RAM maker and have them confirm compatibility with a specific mobo model.
As far as RAM frequency goes independent testing by Tom's Hardware, AnandTech, X-Bit Labs and OCN member "damric" has shown no significant gains in real world applications on typical Intel and AMD desktop PCs with higher frequencies than ~1333 MHz. and even less improvement with tighter latencies above ~1333 MHz. because one clock cycle is shorter in real time as the RAM frequency increases. The reason there are no significant gains is because DDR3 RAM at ~1333 MHz. does not create a system bottleneck as older and slower DDR and DDR2 RAM did. This has all been discussed in the links below for those who are interested including test data from Far Cry 2 in intense RAM usage scenes.
At the moment DDR3 RAM is very inexpensive and 1333 MHz. (PC3 10600) / 1600 MHz. (PC3 12800), frequency RAM Kits seems to be the sweet spot for performance/value if you need/desire to upgrade.
Disregard the personal attacks in this thread which does not change the test results for REAL applications.
: AMD Llano APUs do benefit from faster RAM above ~1333 MHz. in real applications because the GPU gets to use the faster RAM
The reason why I post this information is so that people will understand why they see no significant gain in real world applications with RAM frequncies above ~1333 MHz. By all means run your RAM at whatever frequency makes you happy. Some folks actually Under-clock their RAM to allow higher OC'ing of the CPU - which delivers much better system gains than RAM OC'ing above ~1333 MHz. can.Edited by AMD4ME - 4/22/12 at 6:22pm