|There are some programs that will only display on the designated primary monitor in Windows. E.g. the DVD playback window in Power DVD.|
I don't know if that applies to each card if more than one is installed, or it may display on only one monitor in any case.
In that case if you want it to display on more than one monitor, the only way you can do that is to use a video splitter.
In that case, I haven't tried hooking monitors to each of two recent video cards on a computer at the same time, so I don't know.
I imagine if the video drivers and/or related software are capable of it (in your situation, both cards have a chipset made by NVidia), it shouldn't be a problem.
You can only have two (or more) video cards in SLI mode if your other card is a NVidia chipset card, the mboard main chipset is NVidia, and both the other card and the mboard are SLI compatible. Both cards will work in any case, but the SLI feature will not work unless that applies.
Since you don't seem to have a problem spending bucks going by what you already have mentioned, if you have 2 PCI-E X 16 slots, another GTX 260 chipset card might be the way to go - you would have an incredible GTX 260 X 2 combo in that case.
However, you may need up to a 1000 watt power supply if you were to do that - e.g. a minimum 500 watts is required if you have one GTX 260 card on the system; I'm not sure what two on a system would require - at least 700 watts would be my guess.
You may also need more ram in Vista.