I'm not sure if more monitors will reduce fps in gaming since I'm not a big gamer, but I have a 3 LCD setup with 2 GPUs and it works fine with the desktop stretched over all 3. To set it up for all 3 in XP, have them all connected, go to Display Properties, Settings Tab, and you should see 3 monitors showing up, some maybe grayed out. One should be set as the primary, so it will have the "Used device as primary monitor" selection grayed out, click a different monitor and click "Extend my Windows desktop onto this monitor", change it to the correct resolution, repeat with the other monitor. At this point, they will probably be setup wrong, as in the wrong location. You can identify the monitor numbers by clicking Identify and seeing where they are setup in Windows. Then, just drag the monitor in the settings to the correct spot you want it, side by side or up and down.
I've done this in Vista, so it's relatively the same I guess, but I don't remember how it was done as I don't have Vista installed, I was just played around with it at the time. Not sure what OS you have.