So for example, 3GB of virtual space is enough for most things. In fact, just 1GB is plenty for 99% of all things. But some programs will break, and they can break in surprising ways. Like "my email indexing stopped working" - because my combined mailboxes are currently 2.8GB, and it slurps them all in in one go to speed things up.Heh.
(That wasn't a made-up-example, btw. I had to write this stupid email searcher for the SCO subpoena, and the fastest way was literally to index everything in memory. Thank gods for 64-bit address spaces, because I ended up avoiding having to be incredibly careful by just doing it on another machine instead).