There is an early Flash 10 for G4's/G5's, but it was very buggy with horrible performance and most users preferred previous versions. I also think it takes some digging around to find the version of Flash 10 that "works" on PPC Macs, it's not on the Adobe website anymore as far as I can tell.
You really have to do a few tweaks to get better video performance, but Netflix isn't a possibility because it requires Microsoft Silverlight, a Flash-like alternative that is not compatible with any PPC Mac.
But you can use an optimized browser to make surfing better, and to some extent streaming videos a little better, either
TenFourFox or
Camino, both have specific builds for whichever processor your iMac has (just post what it is and we'll help you with which version browser to download).
You should also go to youtube.com/html5 and opt in at the bottom of the page to the HTML 5 trial. That will make youtube videos that have HTML 5 versions available play in HTML rather than Flash, which is a poorly created plug in no matter what computer you are on. HTML will play in Safari or either of the TenFourFox/Camino builds (I'm pretty sure it works in Camino, if I'm wrong someone please correct me).
You could/should also use
MacTubes to actually get the best performance of Youtube videos. It's a free app. You search in MacTubes for the Youtube video you want to play or download. You can play them using Quicktime instead of Flash (change the settings in MacTubes) and even an older slow PPC can play non-HD videos decently.
Also, download and install
Perian, a Quicktime plug-in that will help play formats Quicktime normally doesn't. It's also free.
Finally, if you have actual video files you have downloaded or ripped, you should use MPlayer to play them. There are some tweaks in the settings to make it play back videos better if you are using a PPC Mac. (MPlayer has a
G4 optimized version, not sure if there is a G5 optimized version, can anyone help with that?)
In short, there is still plenty of life in your iMac, no matter if it is a G4 or G5, but the biggest sacrifice is quality of streaming video. And for most PPC Mac's, streaming HD videos just isn't going to happen, but that doesn't mean you can't play 720p HD files that you've downloaded or ripped, those generally play well on all but the slowest/older PPC Macs or Macs that do not have much RAM, as long as you use MPlayer or Quicktime.
You can find out everything about your iMac by clicking on the Apple logo in the top left corner of your screen, then click About This Mac. If you click on More Info at the bottom of that window you'll get access to tons of information, but the About This Mac window shows what CPU you have and how much RAM, as well as what version operating system you have, (ex. 10.4.11, 10.5.8, etc.). Post all of that here to get more specific help. Make sure you've run all of the Apple Software Updates to get to the most current build of whatever operating system you are on. 10.4.11 is the last version of Tiger, 10.5.8 is the last version of Leopard.
Some other older Mac programs can be downloaded here:
http://mac.oldapps.com/
