The Si 1520 (which is identical in terms of electronics) works with 2GB RAM.
You can install any socket-M-processor to upgrade your CPU (e.g. T5600, T7200, T7400, T7600).
But as faster processors get more and more expensive, I think the T7200 is the highest that makes sense because it is the first one that gives you get the benefit of the 4MB L2-Cache.
Also consider heat-problems with your notebook!
I did some heavy audio-transcoding with mine (took about 6 hours at full load), so I know what I'm talking about.