Yah, you can upgrade just about everything, but do be sure how much money you want to put into it - they're only worth a few hundred now. RAM is pretty easy to find - it's standard PC 133 RAM for most G4 models. I recommend Other World Computing for RAM, as I've purchased a bunch of their ram - fast, reliable, and well priced!
Video cards are a tricky issue - Mac cards are very expensive, but you can flash standard PC video cards with a Mac Rom, depending on the card, chipset, ROM size, and some other factors. (http://theMacelite.wikidot.com/ is a fantastic reference for this!). PC cards are far cheaper, although still a bit expensive. Keep an eye out for sales and at used computer stores - I found a Radeon 9800 Pro for $15 once, and pulled another out of an old Dell desktop that had been thrown out. Note that the QS G4s use an AGP 4x bus, and that due to Apple's special ADC monitor port, you'll need to tape over pins 3 and 11 on any PC card you add. See the link for more info.
I believe you can swap CPUs with other QS models, if you find one used, but you won't get faster than a dual-1Ghz. Beware that MDD G4 CPUs are NOT compatible with QS G4s! You can find accelerators online, but they are all generally a couple hundred dollars, and most are discontinued.