brand-new secondhand iMac - help?

hello, I am a PC linux user, and I decided to buy my little girl an iMac off ebay, the reason being because they look cool (suppose I should've done some research first) well, now I am finding it hard to find information. Maybe some people here might be able to answer some questions for me?

it is a G3 (350mhz) with 256meg, I think it has a 6gig HDD - not sure how to find out

are there some good "for dummies" mac OS websites?

it has mac os x tiger with mac os 9.2 available from within os X or as a boot up option, will it be easier on the computers resources to run OS 9.2 rather than X (like using win98 over XP on an old PC)

I would like it to play avi/mpeg movies from the cd drive, and dvd movies from a usb dvd, is it going to be powerful enough?

I have installed some games and VLC movie player, there is an icon in the applications folder for VLC, but the games are in a folder on the desktop, are these both correct ways of installing software?

also I have a Myson-century 8818 usb external hard-drive, I installed the drivers but it only ever showed up in the system profile once, then never again, and never showed up on the desktop. I think the driver was not the correct one, but I can't seem to find any mac drivers for this device on the internet.

does anyone have any experience with linux on an older imac, is it just a bad idea with all the proprietary mac hardware?