I took the memory from 512MB to 768MB, largely inspired by the printer driver for my newis HP Business Inkjet 1200 dtwn (a lovely printer) taking about 200MB of active memory when priting complex pages. Although it didn't help that I'd left the Visitor account logged in for a month or so -- sometimes Fast User Switching is a bit too transparent. And that helped performance a lot, especially while printing.
Then I got a new 80GB 7200 rpm hard disk to replace the 60GB 4200 rpm disk that it came with. I was always running up against disk space problems, and it was pretty slow. The new one works well and fast, but it does seem to take a bit more battery power and get a bit hotter than the old one.
And finally I bought a copy of Tiger and did a nice new clean install on the new disk, then copied on all my files (which I'd previously copied off onto my external Firewire disk -- shame it's hard to get external Firewire 400 enclosures for 2.5" laptop hard drives). It seems to be running smoothly, but I still have a fair amount of reconfiguring to do.
I also picked up one of these. Once I'm sure there's nothing left on the old disk that I need, I can stick it in this, reformat it and have a 60GB very portable network file server that also acts as a wireless access point and automatically downloads files from USB memory sticks or (with a suitable USB card reader, which I have) other memory cards. It's going to need some configuring to make it play nicely with my Airport Extreme basestation and Airport Express. Should be ready to take to Glasgow next week, where I expect it will come in useful.