Do you already have the 800 RAM? If so, there is no harm in trying it. Most likely it will work but your memory should probably be running slower anyway. The issue is getting the extra 1GB of memory.
Is the memory you have a single stick or 2? Is the new RAM a single stick or 2? You should be using matched pairs instead of single sticks.
Looks like the slots will each only support 1GB each. If you haven't already bought that RAM I suggest you not do that. I believe you can add 2 -GB stick of 667 and get the board to run 3GB total. You need to install the matching sticks in the slots according to the manual in order to run in dual channel mode.
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