mac graphics cards had their own rom so you cant put a PC graphics card one in, unless you can flash the mac rom into them.

Well, here is a post you may find interesting:
This is not a PC, but it may as well be! (My PowerMac G3 Blue & White)

lmttn wrote on Today, 02:43:

Hello, I have a 1999 Power Mac G4 (graphite model, 400 MHz) coming in soon and I have two small questions about it: […]
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Hello, I have a 1999 Power Mac G4 (graphite model, 400 MHz) coming in soon and I have two small questions about it:

• What PCI graphics cards can be used as upgrades over the included ATi Rage 128?
• Can I use any old USB 2.0 PCI card and PC100 SDRAM or is it picky about either of these things?

Maybe. Mac OS X can use NEC and Ali card without drivers. If you have a Via chips on the card... change the card

The upgrades depend on whether Mac OS 9.x or OS X is the OS of choice.

If it's an G4 Yikes with a PCI graphics card, then the PC bus slot is running at 66 MHz. It's like AGP, without GART etc.

The original ATI card probably runs at 66 MHz, while PC cards run at 33 MHz.

Speaking in general, most OS 9 hardware also works on X.
But there are some special cases and a few limits.

A Geforce 2 or Geforce 4 MX can be flashed rather easily, for example.
Many versions have an ordinary EPROM chip in a socket.
The standard timings usually work, though there are tweaking utilities.
Also, it's always good to backup the original EPROM.

For OpenGL 2 compatibility, an Geforce FX 5200 can be used.
But it has no 3D drivers for Mac OS 9 anymore.

Speaking of Quartz Extreme and Mac OS X 10.4 Tiger, there's a workaround to enable it for PCI graphics cards.