I just got a .mac account, and i like it so far. Its integration with iLife is really pretty cool, and it includes virus protection and backup. So I'm pretty pleased with it. I did miss out on the rebate, and it's a hell of a lot cheaper if you buy it from Amazon. From Amazon it comes out to about 5 bucks a month, 60 bucks for the year.
OSX comes with OS9 included with it, and you can install it so you can use older apps before you get newer ones. It's a little weird to get used to - you start up your computer, then run an app - which actually launches os9 in a separate window, and then inside that window you can do things like install applications to be used in os9. It works pretty durn well.
As long as your zip drive has an interface that connects it to your computer, you can use it with OSX, as far as I know. At most, you'd probably need to get new drivers.
In other words, they really do make it pretty seamless to upgrade. The only down side is that if you need to use certain software, and you don't get OSX-friendly versions, you just end up using everything in os9 "classic" mode, which is not necessarily bad, just not as zing-pow! as OSX.
I'm happy to help any way I can. Something about me being a 9 ... anyway, I promise not to pressure you into buying an extended warranty.

BigJohn