Power Mac G4 Quicksilver PCI Graphics Card?

Hi. My brother recently purchased a nice Power Mac G4 Quicksilver. It has a 867Mhz processor, 512MB RAM, 60GB Hard Drive and is really perfect for the stuff he does. The trouble is this. The machine came bundled with a great 17" Apple Display that connects with an ADC cable. The machine can connect to it, because it has an ADC capable GeForce 2 MX AGP video card. Here's the deal. My brother is going to have to upgrade his video card soon, because the video card does not support many of the 3D effects that he uses for his indie 3D game work. I need a card that is compatible with OS9 and OSX. The card I thought would be perfect, is the ATI Radeon 9200. It's got all the features he needs, and isn't too modern to not support 3D acc in OS9. Problem is, the AGP slot is already taken up with the GeForce MX 2, and I can't replace it, because it's the only card I can find that has an ADC output. If I bought an ATI Radeon 9200 PCI card, and put it in the PMG4 with no monitor connected to it, would it improve the performance of the computer, and enable the shaders and pixel effects that it supports? Does it have to be connected to the monitor to be effective?

Thank you.