You can put up to 1GB of RAM in your iMac, so that would be a good start. You might also consider doing a fresh install of Mac OS X to speed it up as well and only install your data as needed from a backup.
Since you're running Leopard on an "unsupported" machine. Disable Dashboard and disable Spotlight to free up your system resources. Install 1gb of memory, 1x PC133 sodimm(laptop) and 1x 512mb dimm(desktop) memory. Too bad you don't have the DDR iMac, 2gb is possible. Also a larger hard drive can speed things up too.