Wait a minute. Are you saying that you're dead in the water on a 2008 intel mac, simply because you don't want to update the OS?
Jeez, here I am thinking you're Lady Copperfield standing in the streets, with a grimy face and fingerless gloves saying "Pleez sir, could you spare a crust of bread, or maybe a computer that's not powered by...owls..."

Holy crap, grrrl, that machine is fatter than mine!

Unless you own a lot of expensive software that is wholly incompatible with OSX 10.x.x, (which you don't, as far as I can tell) then you really should update, anyway. "Migration Assistant" is built in to the OS. I'd recommend backing up important files before you do it, but this is dirt simple to do. Migration assistant pretty much does it all for you.
Garageband will not be affected (actually, moving to 10.x or higher will make your garageband experience even better they probably have more instruments and some new better ones that you can't run yet. I think all of your work will carry over unaffected.)
Yes you may have to update GB, browsers, itunes, etc. after the OS install, but it's like "click the button, accept the terms, reboot."
You'll be able to run at 64bit which will make your music stuff better. Truth is, everything you do will be better.
srsly, a $30 copy of Snow Leopard fixes everything here. At least as I understand your dilemma. Leopard (10.x) was fairly big change in the OS, and it has become the target for minimum OS requirements for many things. I'd also recommend maxing out the RAM in your machine. I think your's can hold 4GB.
Unless you're looking for an excuse to buy a new computer, just find a copy of Snow Leopard and then update your browser accordingly. Add RAM if necessary. Sorry if it sounds like I'm doling out the Tough Love here (rest assured I'm doing it with a smile on my face and love in my heart)
