Even upgrades that are technically possible for laptops generally aren't worth it, with the exception of memory. I'd recommend adding more RAM, and the install process is simple enough that you can probably do it yourself. Anything else in the computer probably isn't worthwhile upgrading, your money would be better spent buying a new machine.

With regards to running OS X, Panther (OS X 10.3) should run fine on your laptop if you're not doing anything too processor-intensive. A new copy of Panther off eBay should run you up about $20.