I think my best all-round experience has been with the OEM Radeon 9000 Pro. That is what I have in my Sawtooth right now. It properly supports dual digital output (dual DVI with the right adapter), supports the highest resolutions on VGA also (unlike the GeForce series for the most part), and nearly never caused any compatibility issues. You also have the bonus that it will work just fine in a dual-boot setup with OS X, should you choose to do that (many other cards will cause you nothing but pain trying to just get a basic boot on OS X). As darthnVader mentioned, passive cooling is also a very nice feature since the machine will run almost dead-silent, and you needn't worry about the eventual failure of the fan and difficulty sourcing a replacement.
If you're concerned about the video card 'out-performing' the 'lowly' G4 450 MHz CPU, why worry if you have that long a list of unused video cards at hand? The less stress on your video card, if your CPU can't feed it data fast enough, the cooler your card should run, which helps the lifespan of every other component in the machine. I also thought that in general the video card in just about any computer was supposed to compensate for you CPU.
Since we're primarily talking about flashing PC video cards here, for most trouble-free experience I would go Radeon 9000/8500/7500 first, accepting you're likely getting a single digital output and a good chance yours has a fan. If gaming were the goal then Ti4600 and a completely different machine with higher bus speed and a lot more CPU horsepower. BTW, Ti4600 runs SCORCHING HOT - better in a machine with significantly improved airflow.