|"I have a Dell TV W4201C/W5001C 42"x50" Plasma TV."|
No you don't. You just listed two different models and screen sizes. The W4201C has a screen size of 42 inches and the W5001C has a screen size of 50 inches. There is no model with a 42 X 50 inch screen size, screen size is measured diagonally and there is only one number.
So which model do you have?
If you have the 42" TV, the native resolution is 1024 X 768.
If you have the 50" TV, the native resolution is 1366 x 768.