You could at least have posted what video card you're using...
According to http://support.dell.com/support/edocs/systems/xpsm1530/en/OM/specs.htm#wp1111453
you use an nVIDIA GeForce 8400M GS.
Have you tried to play around in NVIDIA X Server Settings? You should try TwinView or Separate X Screen under X Server Display Configuration in NVIDIA X Server Settings.
Also: What desktop environment are you using? A light environment might not support two screens that very well and a heavier one might make your 128MB video RAM run out etc.