@BaronKatz,
I have an extensive experience with upgrading old hardware and you can trust me when I tell you that doubling the amount of RAM speeds up the machine at least 10x more than doubling the CPU speed. It is all down to having to swap less: even the fastest storage media are 100-1000x slower than even the slowest RAM so avoiding swapping has a tremendous effect. There is little point frying your CPU at 4 GHz if it spends most of the time waiting for thing to move between the RAM and the slow swap medium.

All those things you list (Falsebook, Google etc) boil down to using more RAM. At some point you run out and the machine has to start swapping. On the N900, that point is rather soon due to the small RAM size.