I am in the process of upgrading the RAM from 1Gb to 4Gb, the maximum the computer can handle.
Even if your computer could handle more, you would have wasted your money buying more and using it (or rather, attempting to use it ... which you can't) on a 32-bit operating system.
Quick question then, what immediate changes in performance should I expect to see from this upgrade?
Depends what you do on your computer ? What drove you to upgrade in the first place ? Boot up might be slightly quicker, menus and stuff might be a little snappier ? All depends how much your computer was paging to disk before.
Vista was never very efficient on memory usage, so you'll probably notice more of a difference with Vista than you would have on Windows 7 or 8.
Obligatory disclaimer..... you might not notice any difference if other factors hit you (e.g. if your drive is full of stuff slowing it down, or if you have a slow processor)