I had a MAJOR perfomance boost doing this thing. My computer had 3GB of RAM (a 1gb stick and a 2gb one). The RAM was operating in asynchronus mode and the total bandwith of the RAM was 5.2gps. I removed the 1gb stick and replaced it with a 2gb one. The RAM is now operating in SYNCHRONUS mode and the max bandwith doubled, it's 10.4gps.
After this upgrade the computer feels like a completly new computer. EVERYTHING is just loading so fast and waiting times are minimum. I LOVE my laptop now
So, my advice, if you have 2 sticks of ram of different sizes, do everything you can to replace one of them (preferably the smaller one) and have 2 sticks of the same size. The system will operate in synchronus mode and it will be FAR MORE nifty and responsive than when operating in asynchronus mode with 2 different size sticks of ram.
I did a test on a friends laptop. He had a CeleronM and 1 stick of 2GB RAM, Win7. It was very slow. I removed the 2GB stick and put in 2 sticks of 512MB each (totals 1GB which is 1GB less than what he had before). The result? The system is way more faster than before even if the total memory dropped from 2GB to 1GB. The limitation of this lack of RAM occurs when runnig programs that need a big ammount of RAM. But this was not the point, the point was that if you put 2 sticks of RAM of the same size, your system will way faster.
Regarding my problem, i didn't manage to get my RAM to run at 800mhz, but i am VERY satisfied with my 80$ upgrade (T8100 + 1 stick of 2GB RAM) Best 80$ i have ever spent!