Installing graphics card on pre built?

A 500w PSU is way overkill for a 7770 unless you're running like 10 hard drives =P. No, you do not have to uninstall the integrated/on-board graphics. When you clip your new graphics card in, the mobo should automatically recognize the device as a video output, most of the time even before the drivers are installed. The type of monitor you'll be able to run games on depends entirely on the ports that are on your 7770 and the type of cables that the monitor uses. But yes, I'd say you'll probably still be able to use a DVI to VGA converter to play on a VGA monitor...assuming your 7770 model has DVI, of course.

It dosn't have a dvi but it has a dvi to vga   (its weird how they colered the dvi blue)

I had a 430w coaisar, would that powered my rig up?