I had an AGP G4 Powermac 400 Mhz that had been upgraded in the following ways:
1. Added RAM to 1 GB
2. Replaced HD with a 60 GB Maxtor (HD failed under extended warranty. warranty company replaced it with an inferior brand. got fed up and bought a new one myself).
3. Replaced my video card with an nVidia Geforce2MX (b/c OS X came out and the ATI Rage couldn't take advantage of Quartz. I believe this card was a flashed PC version).
4. Added an external firewire CDRW burner (original one was only DVD i think)
5. Replaced G4 400 chip with the Powerlogix 1 Ghz G4. (by this time, I gave the computer back to my company, and it was deficient in running Illustrator/Photoshop).
6. Replaced the 17" Apple Studio display CRT with a new 17" Enzo LCD.
This is pretty much the order of how my I upgraded my 1st mac. I suppose you can say it shows the order of priority in upgrade as compared to costs. The last 2 upgrades with the chip and LCD cost about $500 each. Yea, I could have purchased a new computer with all these upgrades. But, this was over a span of 4 years and I didn't have to pay out of my own pocket to do upgrade 6 and 7.
It has plenty of life left. It runs Jaguar very well. Feels a little slower than the G4 933 Mhz Powermacs we have in the office. But it works. It doesn't work well with Panther but I suspect it's b/c of the flashed video card compatibility.