Boy! This is a great community! Thanks for all the advice, guys/gals.
For the record, my G4 is the MDD running at 1.25 with 1.5 RAM.
Again, the things that bug me are 1) The very slow rate of copying files, both within the system and when I copy music to my iPhone, and 2) operations like file conversion: .api to MPG for burning video, for example.
Running scripts on GIMP can be slow, but I can deal with that.
Otherwise, the old girl opens apps as quickly as I need and I can listen to music with iTunes and do other stuff at the same time quite happily.
So . . . it seems the USB card ought to be the #1 thing, then maybe the drive and another 256 of RAM. Yes?