I keep OS X up-to-date on each of my 2 Macs (a G4 Dual 867mhz desktop and an ibook 1.2ghz laptop).
I also have an old PC (PIII 733mhz) that has Fedora Core 4 installed, running in terminal mode, but since it only has 128mb of RAM it really can't run any full scale linux distros (i.e. 2.6.x kernel with the latest Gnome or KDE... they make it choke). It is interesting that Linux, which used to be great for installing on older PCs, now requires a modern, powerful box with plenty of RAM in order to run at maximum capacity.
Oh well, Linux is just for playin'. My Macs are my true love!

KDE and Gnome are pretty bulky when it comes to Linux, but try a lighter window manager such as Enlightenment or my current favorite, fvwm, and it runs like a breeze. I'm using fvwm on a Pentium II 300 MHz, along with several server functions (mail server, web server, file server, etc) and it runs great.

I'd also recommend using a distro of linux that's a little more customized to your system (Gentoo's my recommendation, or Debian is good too), as distros such as FC and Mandrake tend to be a bit bulkier unless you spend a lot of time whittling it down.

As for my machines, I'm running a PowerBook G4 with 10.4, a server with Gentoo, and a desktop PC with Gentoo/Debian.

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Jayson --When Microsoft asks you, "Where do you want to go today?" tell them "Apple."