There is no OS 9x. I assume you mean OS 9, and not OSX(AKA OS 10)
I'm still fairly new to macs, and mine's running OSX so it's hard for me to do too much of a comparison. Of course hardware upgrades would help...RAM, faster hard drive, etc. Some macs offer other hardware upgrades, if you don't mind shelling out some cash. There are accelerator cards and what not that basically give you a new processor. However, they can run pretty high, and the performace isn't at par with other similarly spec'ed macs.
I'm sure there's some software mods you could do, but again, I don't know how to "clean up" a mac as well as a PC...