OK I just took the side off the G5.

The 7800 fan is running just fine without being connected. I also just googled temps and gamers are running at 62+c / 143.6+f I'm running about 122f at idle.

I just keep my G5 with the 7800 GT (and its fan!) under my desk where the extra fan-noise is somewhat muted.

I tried that one, but it did take up the space of two PCI slots. :(

Be aware G5b, you should read the disclaimers for the AllowOldGPUS plug in. It can itself be the cause of system instability.

I think that the plugin works better with PPC machines.

Let us know how stable CS4 proves to be with the two dissimilar cards being used in conjunction with the AllowOldGPUS.plugin.

So far I haven't had any problems with it set up that way I have it at the moment.
I really don't want to spend another $300. for another 8800 card just to get this to work.
Especially when you wouldn't be able to get the full speed out of the 2nd slot anyways...quite throttled down...so why put a $300. card in there.
I'll let you know if I start to have any issues with this set up....otherwise you can assume it's working for me.
Anyone have any info on all those Advanced Settings and what they do and if we need them to do it?

Day two here, and just wanted to report that everything is fine and stable here running a 7300GT and 8800GT card together with AllowOldGPUS.plugin installed in CS4.
Mac Pro (1st generation)
2x3 GHz Dual-Core Intel Xeon
9 GB memory
Two internal scratch disks assigned to CS4 (not start up drive)
Enable OpenGL Drawing activated
8800GT in first video slot running 30" Eizo @ 2560x1600 and a 21.3" Quato IntelliProof 213 @1200x1600
7300GT in 2nd video slot running two Samsung 204B @ 1200x1600 each.
Keeping my fingers crossed....everything seems fine contrary to what other people are reporting. I'm not sure why my set up is working, but I'm thankful!

Because he had two different cards and he thought that would solve the problem of two cards and more then two displays which it at least seems to be the answer.

Other people tried the same two cards on the original Mac Pros and it did not work the difference being he has the plug in installed.

Chris....I had to use the plugin in order for it to work. Otherwise all the GPU Settings were dimmed out and not available. As soon as I installed the plugin, it all worked.
I would prefer not to have to use the plugin, but what can I do...it seems the only way at the moment.

Confirmation:

System --
Quad Xeon 2.66 Mac Pro, 16GB RAM, 2TB HDDs
Photoshop CS4, all panels open and displayed on secondary monitor.

2 video cards - Slot 1 drives a 30" and a 20" Apple Cinema, slot 2 drives an off brand 20" widescreen
Adobe Apps only utilize monitors connected to card in Slot 1. No panels, tools, windows are ever moved to third monitor.

Video configurations and details:

2 x nVidia 7300GT
Open GL active by default but Advanced Drawing disabled (As expected).
Large brushes caused crashes, as noted in Adobe Tech Documents.

1 x nVidia 8800GT + 1 x nVidia 7300GT
Open GL disabled by default.
Installed AllowOldGPUs plug in
All GL features become available, including Advanced Drawing.
Consistent crashing when working with files containing many layers.
Drawing shape layers resulted in a crash 1 out of every 5 attempts with some of my files.
Seems the more layers the more unstable PSCS4 becomes when creating new layers.
Noticeable .5 to 3 second delay when creating layers or committing Layer styles dialog due to layer panel update sluggishness.

2 x nVidia 8800GT
Removed AllowOldGPUS plug in.
Open GL active by default with all features, including Advanced Drawing
Layer creation is instantaneous even on many layered files.
I've attempted to crash PSCS4 several time in the same manner which previously resulted in a crash, but have yet to crash the app.
Overall window performance is better. This is somewhat expected but the amount of improvement was a bit surprising since the second video card is not running at top speed and is primarily used to drive a third monitor not used when working in Photoshop. I'm chalking up the performance improvement to better direction from the Apple drivers no longer being confused by the non-matching video cards.

Here's an update with my situation:

I finally had to remove the 8800 card and reinstall the 7600. It was not causing me problems in CS4, but was causing computer issues with waking up from sleep and major start up issues.
So I'm now back to two 7600 cards and will just have to wait for Apple to deal with the problem of having two different cards installed at the same time.
CS4 does work with the two 7600 cards...it's just a little sluggish for all the cool effects...zooming, tossing etc.

Don, the two 7300 cards probably isn't the issue with sleep. I ran 2-7300GT cards for over a year without issues. I had sleep issues that were due to a corrupt OS at one point. But reinstalling the OS (ugh, from scratch) fixed all those issues.

Ahdissimilar cards. It seems that OSX does NOT like that.
Installing a second 8800 would seem to be your answer when you are able to do it.

Ann....
I really don't like the idea of spending another $300. for another 8800 card. Especially when the other video slots are so throttled down, its a waste of what the card is capable of.
That would be $600. for cards and another $200. for the CS4 upgrade, bringing the total to $800.00!!
I think I'll just wait until Apple gets this figured out, and hope they do and soon.
One should be able to upgrade a video card without being forced to buy two of them in order to get them to work without conflict. Somethings not right with that.

Mismatched RAM will also give you grief.