Then you have a considerable number of options available to you.
Your 4600 may work, but there's a risk involved. There was a change from the MDD AGP implementation to the G5 AGP implementation. I can't recall the specifics, but my ATI 9700 Pro that came out of an MDD wouldn't work in my G5. I suspect you may run into the same problem with your 4600 card.
As for what cards you can choose from, there's a wide range. Sticking just to the Apple and ATI Mac cards, you've got a three types of ATI 9600 cards that would work, two types of 9800s, an OEM x850xt, an ATI x800xt, and a couple of GF6800 flavors. Any of them would give better performance than your 5200, which is at the bottom of the barrel.
I used to run a 23" and 20" monitor on my old G5. One video card--even the one you have now--will run both 23" monitors.
None of the cards will give you great game performance with modern games. Those have passed the G5 by. If you're looking to play 4-5 year old games, you'll be fine. Who wants to do that, though?
That said, you'll have a hard time finding a good source of any of the high-end cards. They'll be expensive for what they do. If you spend a little time learning, you'll be able to flash a PC card at a considerably lower cost.
