UPDATE: I'm writing this from Porteus Xfce 2.0-rc1 on my old Compaq Presario 1670 laptop, 350 MHz w/ 192 MB of RAM in Firefox 3.6.13. It is slightly slow, but quite livable for an older laptop like this one. What I did in order to make it this feasible is I trimmed original Porteus Xfce 2.0-rc1 to a 91 MB filesystem (compressed in xzm modules - it's about 400 MB uncompressed) by pruning out a lot of stuff + sound programs/libraries. Downloaded ff 3.6.13 and made an xzm module of it. It is surprisingly very useful, much more so than less complete browsers such as current midori and epiphany. It works for me....and it has brought this old laptop back to useful life with the functionality of a very modern slackware 14.0 system
For regular Porteus XFCE booted in native aufs, I would suggest 256 MB of RAM minimum.
Post here if you are a new Porteus member and you're looking for some help.