New Toy doesn't work

[G. Klopfenstein]

This morning a huge box had my name on it. Inside was a 27" HDTV. Got my HD box hooked up, looked great. Got my DVD hooked up, looked great. Ran the VGA from the computer to the D-SUB connection(is that another name for VGA?????) and got nothing. Cable seemed to fit, I set the input of the TV to VGA. I looked all over the place to make sure I wasn't missing a VGA input. No picture. When the computer is booting the TV flashes the resolution the computer is set at and then quickly goes to the No Signal screen. Anybody use a monitor this size? Any advice to setting it up? I've had 3 diferent computer monitors hooked up this morning so it's definitely not the computer or video card.

Could be that my video card is that lame? Or is it as simple as changing the resolution on it? I'm pretty sure I've had my computer hooked up to a LCD projector once before. Same thing right?

Mac 10.4.8, G4 1.2GHz, 1.5GB RAM, Video card unknown (because my screen doesn't work and I can't check the specs)
It's what ever the stock one is. The whole point of this TV was to use it as a computer monitor. Help me out tech gurus!!!