I am pretty sure the 7800 GTX 512 was fastest card for G5 QUad.
That depends on the application.
The X1900 easily beats the 7800 in OpenGL (games) while the 7800 GTX 512 is faster in Quartz (pro apps).
The best option is to buy an X1900 since its far cheaper, OpenGL performance is so much better and Quartz performance difference is minimal.
X1900
Quartz Graphics Test
163.04
Line 129.70 8.64 Klines/sec [50% alpha]
Rectangle 158.03 47.18 Krects/sec [50% alpha]
Circle 142.52 11.62 Kcircles/sec [50% alpha]
Bezier 142.56 3.60 Kbeziers/sec [50% alpha]
Text 384.75 24.07 Kchars/sec
OpenGL Graphics Test
126.47
Spinning Squares 126.47 160.43 frames/sec
User Interface Test
142.96
Elements 142.96 656.13 refresh/sec
7800GTX 512
Quartz Graphics Test
165.55
Line 132.24 8.80 Klines/sec [50% alpha]
Rectangle 155.06 46.29 Krects/sec [50% alpha]
Circle 144.67 11.79 Kcircles/sec [50% alpha]
Bezier 146.57 3.70 Kbeziers/sec [50% alpha]
Text 407.15 25.47 Kchars/sec
OpenGL Graphics Test
94.66
Spinning Squares 94.66 120.08 frames/sec
User Interface Test
150.93
Elements 150.93 692.70 refresh/sec