You cannot upgrade the graphics in any iMac (unless the ones out today are very different). You do not have a graphics card. The graphics hardware is part of the logic board.
Yes. I went to a Apple store to purchase mine. It really made a huge difference from the stock card. I have the Revision A Powermac G5 at 2Ghz.
Even with my G5 maxed out, it still can't beat my MBP with encoding performance. The MBP is twice the speed for H.264 encoding. But the Powermac G5 gets much better frame rates with Quake 4 when both are using Universal binaries. This is because the graphics card was so much more powerful.
A rev A PM G5 has AGP inside. The fastest Mac upgrade card for it is the Ati X800XT. Or you can look around at Strangedogs and they can flash an agp pc card for you.