First off, what are you going to do with this chip? Are you testing a video display device? Are you trying to show an image or text on a screen? Or do you have some other use for it. Or do you mean a full video controller chip such as what's on a typical video card?

Video sync patterns can be generated directly by a standard 8051 micro up to 1024 x 768 at 60Hz refresh. If you use one of the Dallas single cycle parts (such as the 89C420 family) that limit goes up to 1280 x 1024 at 75Hz refresh. I have built and programmed many such devices to test our video sync adapters.

Of course, the micro can't handle the pixel data which, at 1280 can be as high as 150MHz. If this is a text application at 640 X 480, for example, it is possible to do the whole thing with a fast micro and a character generator ROM. After all, the original "Glass Teletype" was built entirely with TTL and 74C CMOS parts.