RAM query

Mate who hasn't much money came to me with his desktop a "getting on" Compaq Presario SN5540AN with an AMD Athlon dual core 5400+ – 2.8Ghz Vista (+SP2).
He said to me saying that everything they tried to open prompted a you have a virus message and nothing would open.

Usual sort of clean up – temp files, Ben Hur's cookies, and updating Windows (tons) and had not updated the security for a long time (cannot get into the set up tools to see what has been set). Security is Trend Titanium by the way.

Now it's going a bit better but still slow even after installing a stick with 2Gb Readyboost but think it's worth the installing of 4Gb internally instead of the 2Gb it's got now, at least it's not hanging out of the machine and liable to being pulled out etc.

Currently got 2x 1Gb Samsung DDR @ (800 MHz) what I want to know is it worth going to 1066MHz sticks or not, or is this going to be governed by the mobo?
Mind you the 1066MHz are about twice the price of 800MHz

Use is just usual stuff emails school work any help really would be appreciated.

Have found the mobo "model" – MCP61PM-HM (Iris8)

Thanks Ignazatsonic Stormy and Shootist yeah I found the mobo info and it only supports up to 800 abnyway eh?

So will go with the two sticks of Corsair VS2GB800D2 2GB PC-6400 DDR2

Thaanks so much for your time and patience I know I can be trying at times.