1. The graphics card has no noticeable influence on 1080p playback. The 2.0 GHz G5 can play 1080p MKV rips (15MBit/sec) since VLC 2.0 (and CorePlayer, but this software isn't free and completely unsupported by now).
2. The X1900 is the best value for money card for the late 2005 G5. I have successfully flashed many cards from 7800GT to 7800GTXs and various X1900. I can confirm that the x1900xt 512MB and X1950XT 256MB can be flashed to the G5 version. I always wanted to test the x1950pro since there's an abundance of them in Europe, but didn't manage to get ahold of one by now.
3. For Airport/BT support, I used a 3$ BT dongle and a mini-pcie card with wifi antennas, popped in a standard apple broadcom card and have full airport support since then. cost me about 40$, way cheaper than buying the standoff card and the combo card.
The ATI X1900XTX is a 512MB gfx card, I thought that the one that is compatible is the 256 because there is no existing 512MB ROM?