I was snooping around, and I found out that the Nvida (or however that's spelt), FX5200 and it's derivitives work PnP out of the box on a mac with core image. That is great news, as no other PCI video card I know of does that. However, I was planning on getting a ATI radeon 9200 Mac edition PCI with 128mb VRAM to boost video performance on my B&W. It is nice because both OS 9 and OS X support it. In fact, it is the best PCI video card for OS 9, just like the radeon 7000 is the best card for system 7.
However, I am running on a single display here, a 15" Apple Studio Display LCD, model M4551. Is there a video cable that has 2 male jacks and a single female jack, VGA on it? Or is this just another one of my impossible pipe dreams that I seem to do a lot of?
-digital 
Are you asking if you can switch between the two video cards without rebooting?
You can get some of that functionality using a KVM switch, but the unselected video card at boot time will appear as a second monitor to the OS - if it appears at all. Macs sometimes have problems seeing a second video source if it's headless at boot time.
The differences between the two cards aren't worth the fuss, IMHO. Just get the 9200 if you want 3D support in both OS 9 and X.
What I want to do is have one video card in one of the PCI slots, and the other one in another PCI slot. Then, have some special cable hooked to the VGA output of both cards, and that output would go in to a single monitor. Perhaps have some way of disabling one of them, and enabling the other, or vise versa.
The FX5200 would be for OS X use, as I found out is supports core image after PCI extreme is applied. I assume it also supports Quartz Extreme. Then, I flip a switch, reboot, and I am in OS 9 with the Radeon 9200.
hope that explains it better.
However, perhaps someone could tell me if there is a way to make the FX5200 work in OS 9, then I would not need the 9200. By work, I mean better than 640x480 on 256 colors, which is what I have read about here. That's the page about core image with the B&W G3, the machine I want to do this one.
so, essentially, if there is descent OS 9 support for the FX5200, then there would be no need for the Radeon 9200.
-digital
If you're rebooting to select video cards, then yes, a KVM switch will do what you need. Switching to the other card on the KVM at reboot will ensure the OS senses the card sensing the monitor and is selected as active and primary.
That said, I still think what you'll gain is minimal. The two cards are both, at least on the PC side, considered mid to low-end when released. Core image might help in some areas, but the benefits of PCI Extreme have some caveats, like a demand on the processor that can outweigh the benefit to video depending on the task.
You might find more speedup in buying a Sawtooth for its AGP slot, but if this is a "because I can" hack, then by all means, go for it!
Yes a KVM switch would be what you are looking for, with a KVM switch you could hook up a few diffrent computers at the same time depending on how many computers the KVM supports.
for what you want to do you would need a KVM that supported 2 machines, with that you could run a cable from each VGA port on the 2 video cards into the KVM switch and then hook the monitor to the KVM. also the KVM stands for keyboard video mouse, which you can use one mouse, one keyboard, one monitor to use for a few machines all running and all you have to do is move a switch or do a key combo on the KB to select the mode you want which will switch to the input for a different machine.
if you ever watched the screensavers (before they got axed ) that is what they used all the time for the monitor/s on the desk.
http://iogear.com/main.php?loc=product&Item=GCS632U
that might be something you are looking for, beware most KVM switches are expensive. the one linked above i think you can buy at walmarts
I've got one of the Iogear KVM switches with dual PS/2 connectors rather than USB. Found it at a thrift store for $2. I've been using it with two PC's. I tried hooking up one of the VGA connectors to a B&W once but it didn't work--just got a black screen. I assumed it wasn't Mac compatible. I see in the link provided by madmax that the illustration shows a Mac tower and a PC hooked up at the same time, so the USB version must have no problems with Macs.
Another option, and maybe better option, would be to get a monitor with dual inputs, right? Most LCD's come with both DVI and VGA inputs these days. I've got my daughter's computer hooked up to a somewhat old Sony 19" Multiscan G400 flat screen CRT (found it discarded, sitting by the trash at another elementary school, looked a little dirty, but I decided to take the chance anyway and took it home, cleaned it up, and it works great! I love cheap and free stuff--lots of free perfectly working CRT's these days because everyone's switching to LCD). This Sony's got a sliding switch on the front which allows you to switch very simply between it's two VGA inputs. Very cool.
So which video card should get the honor of sitting in the 66mhz slot?
I use a Sawtooth G4 on my 4-port KVM PS/2 switch. This was accomplished by using a USB-PS/2 adapter (around $7-14) and plugging the KVM cable into that. Works like a charm. It also works with a Blue & White G3.
While I do have a Beige G3 and a 9600/300, I haven't tried using the adapter with an add-in USB card on either of them yet. The G3 is sitting at a desk in my office running 8.6/9.22 playing Classic games (Dark Forces! woohoo!) and providing A/V capabilities for my Dreamcast.