whenever ive had a slow computer running OS X ive turned off spotlight (10.4), turn off dashboad (10.4), window shadows (unsanity haxie), tunred on PCI extreme (PCI based mac), scale effect instead of genie for window minimizing, turn off dock animation, soft overclocking the video card
when installing the OS, omit things like extra printer drivers, language translations, fonts to save HDD space. 6GB can fill up fast
if ur feeling daring there's a jumper block for overclocking the CPU multiplier under a void warranty sticker on the motherboard. u can use small 1mm jumpers to overclock it. at the very least u could get a few more MHz out of it
more RAM will make the OS faster more than any small tweaks
every little bit helps on slow macs