Text displays usually have a driver built-in that lets you stream character codes through a serial connection. These are nice because they require very little data from the controller.
With graphical displays, you're sending pixels instead. The format can change depending on the display you're using. Normally, it "scans" the screen like a VGA monitor (left to right, top to bottom), but it splits the screen into two halfs. Each byte you send in writes four pixels on the top half and four on the bottom half (to cut flicker). This means you need to use some logic or software to dump your framebuffer in order.
Is the 89C51 a Microchip PIC product (it sounds like one)? If so, it's probably fast enough to dump memory out onto a small screen that uses the format above, but might not have time to do anything else in between (like draw the text / graphics on your framebuffer). It really depends on your application and how big the screen is going to be...