http://www.crucial.com/uk/systemscanner/That link should tell you all you need to know. You don't have to buy from Crucial though, just note the types of memory it recommends.
Looking on MSI's website you can have a maximum of 8gb DDR2-800 or DDR2-PC2-6400 (same thing, different description). Only buy DDR2-1066 PC2-8500 if it's cheaper, as it is backward compatible but will run at the slower PC2-6400 speed regardless (your motherboard only supports up to the PC2-6400 speeds).
If you're adding to your existing memory then your new sticks will run at their speed as a maximum, so again you need to bear this in mind.
