I just took a GT240 out of my PC because it kept crashing the machine, so I can't look at the settings now. (It seemed it occasionally tried to pull more power from the PCI-E slot than the slot can deliver).
I replaced it with a GTS450, which has a separate power lead to it and has been fine (so far).
The Windows setup panel for the 450 seems to imply that it will handle two monitors. I don't have two here at the minute so I can't try it to see.
I had a "test" machine with Ubuntu, and had terrible problems with the graphics setup. When the old card died, and I fitted a different brand, Ubuntu wouldn't play at all. The video driver process seems to be very tortuous.