I'm running a Radeon 9600 in my Quicksilver. I'm currently using the 9600 Pro PC and Mac edition, although this card can sometimes be touchy in Quicksilvers.
The 9600XT is probably more readily available and is(usually) less expensive. This was a BTO card for G5s, but will work in a G4 if pins 3 and 11 are taped. A little bit more modification will allow it to be used with an ADC display.
On paper, the XT is a better card than the Pro(other than the pro having 2x the VRAM), but my experience in the Quicksilver shows them to be about equal.
9600s support Core Image. 9700s and 9800s are also somewhat popular choices for some of the other G5 users in here.
Any card that supports Core Image will also support Quartz Exteme. Your current GeForce 4MX supports Quartz Extreme.