The graphics card will not affect the performance of the Power Mac G5 (at least, not when you're talking about the difference between the 9600 and the X800XT) unless you're doing things like playing high-requirement games. In that case, getting a lower processor-speed G5 with the better X800XT graphics card is the better choice. You're not getting a whole lot more with an extra 300 MHz, either, on the higher-end model.
FYI- The G5 requires 8X video cards - 4X cards will not function.
MacBook Pro (15-inch, Mid 2012) 2.3 GHz Core i7, 16GB, 256GB M4 SSD
iPhone 8 - 64GB S/G • Sound System Audio Engine A2 • Display UltraSharp U2412M 24"
Custom Built Gaming PC Kaby Lake Edition | 16GB DDR4 | GTX 1070 8G