Silly question, why did you bother to talk to Dell Support in the first place? you must have 50+ people on just this forum alone that could have guided you through the process with far less hassle, I wouldn't be at all surprised if the video card was fine but wasn't sat in it's slot properly & that's why it wasn't working. Opening the case doesn't invalidate any warranties even if they try to tell you it does.

Windows 7 64Bit
Poser Pro 2010 SR1