Well Mayo, I'm thrilled to hear that you were able to upgrade a 600 Mhz G3 iMac to Tiger! It was the consensus here at TS when I went to upgrade the OS on my daughter's 500 Mhz G3 that 10.3.9 was as far as it would go... so maybe that extra 100 Mhz speed bump was the ticket!
But in reference to this thread, Ryan, I did basically the same thing you did to your Mom's computer to my daughter's Dalmation iMac, a very similar machine, in that all the bus specs, G3 speed, drive size, maximum RAM, etc., are exactly the same architechture.
We found a noticeable increase and smoothness in some applications, but IAW XABD that your video card and 500 Mhz G3 processor are not going to be very happy running internet videos... you could get by with the 100 Mhz system buss, but that 500 Mhz processor just isn't going to run some things that smoothly. The only caveat that I'm aware of is whether you may have had enough applications open to negate the amount of RAM you now have in that machine. But I do think that the basic problem with running that video doesn't have a whole lot to do with RAM.
I upgraded the RAM in two installments, going from 256 to 640 with Jaguar, and then adding another 512 stick when I upgraded to Panther. Noticeable in some apps, negligible in others, and it's been long enough that I played around on that computer that I can't remember which ones
I have spent hundreds of dollars trying to stay "current", and try to keep several older machines upgraded so that I could "keep up" with present technology. The biggest drain was my old 9600, which predates your mother's iMac by several years. I added a very expensive G4 upgrade card, all kinds of stuff, only having to pretty much junk it a year or so later because it happens to be one of the 9600's that's too fickle to reliably run OSX with Ryan Rempel's wonderful program, XPostfacto. There are only a handful of TS'ers who are still using OS 9 on a regular basis. I still boot to it occasionally, but only when I need a program that won't run in Classic (which the new Intel's don't support).
It's only a matter of time before the G4 architecture becomes obsolete. It will probably be a few years yet, and IAW Mayo that there are some pretty good deals on some fairly fast machines and some G5's that are much faster. The G5 architecture is a huge speed bump from the G4, but I expect to be doing just fine with my G4 iBook for quite a few years to come. A fast, relatively cheap G4 machine might be fairly cost-effective alternative for you or your parents to consider...
Don't feel too bad about your recommendation on the RAM upgrade, it was a logical step, and some stuff will really work much better.
HTH,
Chris K