Well, you can add a custom resolution to match 1440x900. If you're using the old nVidia CP, then click on the Screen Resolutions thing on the left, to bring up the screen resolutions settings. Then, add a custom resolution of 1440x900.

If you're using the new nVidia CP, then I haven't a clue since I've never used it, but maybe someone else who has can help you there (if this is the case).
You won't need to buy a new video card, but, for World of Warcraft to be run at 1440x900 with all highest settings would probably require at least a GeForce 6600 or Radeon X700. If you don't mind the settings not all being set to highest, then your card will be just fine.