Well, the success I've had with doing extended desktops has been with an ATI card, but I'll see what I can recommend. After getting the card to see and display to both monitors using the ATI software, I believe the next step was to go into the KDE Control Center. Since your card is already aware of and displaying on the second monitor, try Peripherals > Display in the KDE Control Center. For mine I believe I had to go to the Multiple Monitors tab, enter the height and width of Display 1, the laptop screen, (if they weren't already there), then enter the width of Display 1 as the X Coordinate for Display 2 and update its height and width if necessary. I believe that after doing that and applying the changes, the Screen Size drop down on the Size & Orientation tab had been updated.
I have some notes elsewhere on the steps I took, so I'll see if I can find them. In the mean time, I know there are some other threads on these boards that led me to the steps I used, so you might be able to find them if you search for phrases like "x coordinate" and such.