As VisionRay said, make sure that before you buy a case, you need to find out if the form factor of the HP motherboard is a standard format, or proprietary. I know that some OEM's have proprietary form's, and you won't be able to get the hardware in without modification to the motherboard tray.
Rather than call HP support, you could probably call Best Buy, Circuit City, or CompUSA and see if they can confirm if your model PC has a standard form factor motherboard.
If this is your first time putting hardware in a new case, know that you can NEVER ASK TOO MANY QUESTIONS... better to be sure and doublecheck than be sorry, and lose a component because you missed a step.