Heh. Gave us the Gigabyte card's link for the motherboard.
Aside from adjusting the mobo's XJ1-5 jumper, check to make sure that the BIOS is updated to version 0901. If ASUS ever updated the board's PCIe-2.0 compatibility, updating the BIOS should ensure you have it. If needed, you can download it here: http://www.asus.com/Motherboards/Intel_Socket_775/P5GZMX/#download
The fact that the PCIe x16 slot is only capable of running in either x1 or x4 mode means the card's performance could end up severely limited. If the card should work, I wouldn't expect it to breathe too much life into the system in terms of gaming performance, despite it typically outperforming the on-board Intel GMA950 tremendously. This would be especially true if it were only capable of operating in PCIe x1 mode.