AMD64 and memory explained
The AMD memory controller
The memory controller on AMD64 Athlon / Phenom / Sempron CPUs is contained in the CPU itself. It doesn't matter what mainboard you have, Nvidia, ATI, or ULi chipset based, mainboard chipset is irrelevant to what memory is supported, and any limitations are not due to MSI either, you'll find the same limitations on any other manufacturer's mainboards.
For AM2/AM2+ systems using DDR2 memory
AMD Athlon64/Sempron CPUs support a maximum 800mhz memory speed.
AMD Phenom CPUs support a maximum 1066mhz memory speed.
For AM3 systems using DDR3 memory
AMD Sempron and Athlon II X2 CPUs support a maximum 1066mhz memory speed.
AMD Athlon II X3 & X4 and Phenom II CPUs support a maximum 1333mhz memory speed.
So can I use faster memory?
If you buy memory rated to run faster than these speeds (listed earlier), then you will have to overclock your system in order to use your memory at its rated speed.
Remember though, when you overclock, you are exceeding specifications of your memory controller inside the CPU. An overclock is never guaranteed to work...
Of course, these sticks will happily run at lower speeds.
Please, keep overclocking questions out of this area, but post your question here: >> Overclockers and Modding Corner <<
What memory to buy?
If you are not planning on any overclocking, then any generic DDR2/DDR3 memory should work.
To get the most out of your memory, use two sticks in dual channel mode. If you use 4 sticks, then your memory will work, but only at a lower speed.
If you are looking for performance memory, look for Dual-Channel kits tested for AMD systems, as many are Intel optimised, and the Intel timings are not always suitable for AMD systems. Don't waste your money buying Tri-channel kits, they are optimised for Intel i7 systems, and you'll get better performance using just one or two sticks anyway. And if you're lucky to get three sticks working together, they'll only run in single channel mode, which will hinder performance anyway.
Don't mix and match sticks either. Always use identical matched sticks for best performance in dual channel mode.
For optimal results and trouble free maximal performance it's recommended to use 1 memory stick per channel, as a general rule this always works.
4gb+ memory
Due to 32bit physical address limitations, if your system has 4GB or more RAM installed, a 32bit operating system will only be able to utilise 3.2-3.5gb, because of memory reserved for other resources, such as VGA cards, RAID controllers etc. It is pointless having more than 4gb RAM in a 32bit system.
To be able to utilise 4gb+ memory, you need to run a 64bit operating system. But while the 64bit OS will be able to 'see' and utilise more than 4gb of RAM, any 32bit programs will still have the same physical address limitations, in order to fully utilise more than 4gb of RAM, you'll also need to utilise dedicated 64bit software also. So again, it is also pretty pointless to have 4gb or more, in a 64bit OS, if you're only ever going to run 32bit software.
For more on this, look here: http://forum-en.msi.com/index.php?topic=108883.0
With regards to DDR3 memory beyond 1333mhz, have a read of:
http://forum-en.msi.com/index.php?topic=149100.0