If you are unaware, as I was until today, of the existence of the Minimo project, it is an embeddable web browser based on Mozilla's Gekko engine.
An interview has just come out with two of its developers and they mention that they ran the software on a Zaurus in 32MB of memory. That would suggest a 5x00 machine but they apparently use Familiar Linux rather than any of the usual ROMs (the browser needs GTK).
Reader comments section includes a brief discussion of porting the code to PalmOS with one person noting that it is doable but a lot of work. The same would hold for Qtopia/OPIE, presumably...