If the G41 mobo is already there, I guess you could get a Q6600. Can't say for sure until you name the exact model though.
As already said by others, all PCIe graphics cards will run with it. If you want to be absolutely sure about compatibility, go with the HD 7000 series (AMD) or GTX 600 series (Nvidia) at max and make sure the PSU has enough power for it.
Also try to get a CX450 or CX550 PSU, they're newer models than the CX500 and they're better quality - shouldn't cost more.
Last but not least: Please turn off CAPS LOCK, IT'S KINDA ANNOYING IF YOU GET WHAT I MEAN. (<- just showing it, not shouting or anything 😉 )