A little more info please, how did the problem first start ? Did you put in a new video card and then had no picture or did the graphics just stop working ?
You have already tried replacing a number of components and presumably these replacement components were known to be working before you fitted them ?
Before looking at more complex possibilities, have you eliminated any possible fault with the monitor you are using ?
When you first switch the computer on, do you hear any beeps ?
Learning all the time.