It depends on what you are using your machine for and what version of Ubuntu 10.04 you use.
Is it the 32 Bit Version? Then you can't break the 4GB barrier (see: my response here). Is it the 64Bit version: Then you CAN add more ram. But do you need it?
If you are using your machine for surfing, office, etc: I don't think you need more RAM.
You are manipulating complex graphics, you develop software, you are running games: MAybe you should upgrade.
Why upgrading does help in general was already written down here a few times (run more (background-)apps at the same time, single apps can work with more data without swapping). You should also consider, that modern OSes (like Linux) use unused RAM to cache data for faster disk access.