This has been discussed in the past in several forums and tha answers seems to be:
About adding more RAM and how it does "increase speed" in computers:
You may wish to take a look at this answer: http://askubuntu.com/a/125169/9598 which explains what increasing memory will do in your laptop.
And about the need to reinstall the system if you add more memory:
Basically no. Ubuntu will notice that the system memory (RAM) has been increased and will manage to use the total amount of your memory in the first boot.
As I mentioned before, the system will notice "amount of RAM increased", not "speed increased". This will be reflected on multitasking instead of "high speed"