> 1. Seems like the best overall card for sheer performance
> is the Radeon X1950Pro. Is that true?

If you want 3D support, then yes, an off-the-shelf Radeon X1950 Pro should be the fastest. Note that the OpenFirmware console and boot menu cannot be accessed with such a card that lacks a PPC-compatible ROM.

> 5. How has UAE evolved?

Latest development:
https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=12242&forum=12

> 8. How fast is the G5 at 1.8 GHz with a fast video card?
> Can you play HD video smoothly?

720p should work without problem. 1080p performance depends on codec, profile, bitrate etc. 1.8 Ghz G5 probably drops quite some frames when playing usual 1080p videos from Youtube etc.