First of all it depends on your G5 model, supporting anything from PCI/AGP, over PCI-X, to PCIe (latest model). Secondly, would you like to have an ADC or DVI connector for your display? Thirdly, your usage.
In terms of raw professional power and overkill, it is a NVIDIA Quadro® FX 4500. But this is high budget and only useful if your are into professional rendering and modelling. It is not a gaming card!
As for powerful every day use, I suggest either the ATI Radeon X800 XT or the GeForce® 7800 GT. Although, recommend the ATI in this case for better drivers and power.
Please check the specs regarding the connection issues though before jumping on any.