Raymond has posted several posts about the /3GB switch and about how it affects the Kernel. This is a frequent topic of discussion for highly scaled Exchange 2003 servers due to the recommendation that most servers with >=1gb RAM have this switch applied (see KB.823440 or KB.810371 or KB.328882 or KB.266096 or... well, that's enough for now; there are a bunch of KBs on the Exchange memory management topic.)
Since Exchange is such a memory intensive app we talk about this a bunch. I think it's critical to understand the virtualized nature of the way memory is used on modern operating systems, and Raymond hits it on the head here.
Short version: You can run the /3GB switch on a machine with 128mb. You can run a machine with 4gb and not run the /3GB switch. There's absolutely no connection. The /3GB switch controls the allocation of the virtualized memory space provided to each process (which is generally 2gb per process, regardless of how much physical memory is present). All you're doing when you specify /3GB is changing this 2gb virtual space to be 3gb. Will you ever use the full 3gb on a machine with 128mb? Let's hope not! Your pagefile would be going crazy if you did!