I once thought that 2GB is better than 1,5GB RAM, but in what I do I noticed, that RAM usage goes never over 768MB or 900MB at max. I thought that it would help in buffering data, when encoding videos, but there is no difference running my Mac with 768MB or 1,5GB or 2GB. (I had no or similar to none page-outs/ins. I had to open several new applications and windows, to get it do page-outs. But that always just showed a quick and short rise, that didn't persist)
I guess people that use Photoshop heavily may have some good experience about what more RAM does. (OK and music production). But for everything other as long as you have 768MB you are good. (People say that TenFourFox is a memory hog, I never looked for that, so maybe this could be another area, where it could help, when using 10+ tabs or so).
On the different models, they all have adavantages and disadvantages (for example heat modding), of course.
On the Dual 1GHz QS, I think, most people will get a Single core Sonnet upgrade, since they are the most common on Ebay, so a Dual 1GHz QS will still be cost effective against a CPU upgrade.
Just for fun, here is an article that links benchmarks to upgrades
http://beta.ivc.no/wiki/index.php/PowerMac_G4_Upgrades (I didn't find the article I was actually searching for, though... but it is still interesting)