RPS - let me put some of yours and your dad's worries to rest.
1st - you're probably not going to have enough room to do partitions on the old imac you mention in the other thread. don't worry. it works fine with it all on the same partition. in fact there can be disadvantages to partitioning. you're probably goin to want an external HD before long anyway if you don't already have one. and that will give you plenty of options.
2nd - having used macs since the mid 80's, i can say that most of os x is just like mac has always been. don't let the 'nix terminal jockeys scare you. i've only used the terminal once and i manage to do everthing i want in os x. that being said, the thing that will cause you the most confusion are "permissions". not to worry again - there are LOTS of threads here that deal with how to use them once you run across a problem with them. no big deal. You will probably want to go to
www.versiontracker.com and get an app called ASM which brings back the application switcher if you have a habit of reaching for the upper right hand corner to switch between apps. of course, the dock will work for this as well. just a matter of habits.
3rd- lots of free and shareware apps at versiontracker - it's a virtual playground of fun stuff. some of the apps you have now will have free os x updates. others you'll have to pay to upgrade. you can continue using almost any app in classic - where you run os 9 at the same time as os x. i can only say this is a really cool thing when first switching.
4th - os x is incredibly stable, so there's not a lot of need to "mess around" like you did previously. in fact there is almost no reason for you to ever open the system folder. anything else can be screwed with pretty easily and then fixed, just like always. (of course it's smart to back up things you play with)
tell dad a 44 yo mac user who started with a 512kE in about '84, thinks os x is the only way to go and that once he tries it, he'll want it back!!
remember the best point - you don't give up os 9 to get os x, you get both!!