For gaming it will default to 1 monitor view and then you can setup dual monitor for most games of the X850, and then for something like M$ flight sim it will allow you to assign monitors but it's a bit more complicated , but also more basic rendering.
Just to make sure im understanding. It is possible to use the 3rd monitor (pci slot) for gaming, while the Video card in the AGP slot does the processing necessary for gaming?
Im playing Bf2, im not interested in mulitple monitors for that game. one for the game, one for TS, etc....and the 3rd if for the projector, which is what i may want to play bf2 off of every now and then.
The 3rd monitor is possible on for very specific gaming, like M$ FlgihtSim, everything else it won't work in anything but stretched LOD, and you'll likely have alot of problems doing that. So think of it as no triple-monitor gaming for antything other than Flight Sim.
As for running other apps alongside gaming that's very game dependant, some work well other not so much (Oblivion has alot of trouble from what I've been reading from feedback).
You'd have to check BF2 forums for more info really. Right now I only play Oblivion (barely do that) and I don't even bother with multi-mon while gaming, only for video/picture editing.
Definitely possible to add it. I was trying for a very similar set-up to what you're describing here earlier. Be careful on your PCI card selection, I had an AGP 6800GT and a PCI FX something or other, and they did not want to play nice together. Not sure if Radeon has the same issue, or if my incident was isolated. I ended up selling the card and just hooking up two monitors to my GT.
I agree with ape, but would like to add that trying to play a game like oblivion on a pci card will be difficult at best. Your best bet is to just hook up the projector to your primary video adapter when you want to game on the projection screen, and otherwise have it connected to your pci card.
Personally, I would look for some kind of switch that allows two vga imputs and one output. Maybe even a cheapo KVM? Don't know much about those, so I don't know if it would work.
Another thing to check: Does your projector have multiple vga inputs? Perhaps it has some other kinds of inputs that you can use for the gaming rather than the straight VGA adaptor?
Just saw C4's post and it reminded me: Make sure it is the same company, eg ATI or Nvidia. Otherwise you could run into some pretty nasty driver problems.
You could get a PCI card that has an svideo output and use that normally. Then what you would do is get a DVI splitter that splits one into two (inexpensive if I recall) and plug that into the projector. Then you could just switch inputs whenever you wanted to play on the "big screen." I'm gonna try to find a link for a splitter like what I'm talking about.
Edit: This is the sort of thing I was thinking about. But I guess I was wrong. Nothing that I could find on newegg was what I was talking about. I guess the only thing you could do (or at least the least expensive) would be to physically switch the cables every time. Just make sure its accessible for you.
On another note, instead of SVideo, you could use component video. There are several adapters in the 20 dollar range that adapt DVI to component video. Just make sure to use high quality component cables. Using normal cables can make a big difference in the quality of signal transmission. Don't ask me why it is that way, but I have personally seen the difference, and it is substantial.