The 9800 card is made to work with OS X, (in fact there is no support for 3d acceleration in OS 9 with this card), they do however have some updated drivers and additional software to install.
Yes, ADC is apple's own display connector, and used for their displays. It is basically a dvi connector plus it also supplies the power to the monitor as well, forgoing the need for an extra power cable for your monitor.
If you do end up getting a flat panel monitor later, just make sure you know what type of connector it has. They can come with vga, but you will suffer some loss in quality vs dvi. And there are some companies out there that make LCD's that are compatible with Apple's ADC, so just verify what your looking at before buying.
The GF 4Ti was (still is for now) the top of the line video card based on nvidia's GPU, that you can still buy for a G4. Here is a link that shows some tests run between the GF 4MX and the 4TI.
http://www.xlr8yourmac.com/Graphics/geforce4_ti/geforce4_titanium_Mac.html
The 4TI really pulls away at the higher resolutions, vs the 4MX. Here is another link to all of the video card tests that have been run at this website. He has compiled some amazing results/data.
http://www.xlr8yourmac.com/video.html
This test is just what your looking for when as far as the difference between the 9600 and 9800.
http://www.xlr8yourmac.com/G5/Dual_g5_9600_9800_tests.html
Like I said before, if you aren't concerned with needing a card with an ADC connector, then the 9800 is the way to go, you can't go wrong with this card. In fact you will (like myself) run out of processor power to feed the card before you will ever max out the card.
Enjoy.