Hello finiduck,
There are certainly many people who are much more knowledgable than myself, but I'll start the ball rolling. Ever since Windows XP came out, I have been of the opinion that light usage of the computer (web browsing, word processing) requires 230-240 MB to run without unnecessary paging. Memory has been cheap enough that I have always urged 256 MB for Windows XP systems.

The most important size of a program is the 'working set' of pages that it should keep in memory to avoid extra paging. That is less than 100 MB for either of our projects, even for our largest project, FAAH. So even 384 MB would allow a computer to run FAAH at full speed while doing all the normal routine. (At least, no slow down due to memory constraints.) With 512 MB, you can run some serious programs and still expect that the project can quickly take advantage of any spare CPU cycles. Obviously, it all depends on just what applications you are running.

But do keep in mind that we use 300 MB of Virtual Memory, which is much more than non-scientific applications normally use. So make sure that your Windows XP system is set by default to expand the VM automatically if required, because a big application could run your VM past its current setting.

This is my opinion. Anybody want to chime in with theirs?
mycrofth