Hi, welcome to tom's. I do not know if you can upgrade your video card. Apple has a weird upgrading thing if i remember right. You might just want to sell your PC and buy a costume made one. We can help you with that. If no, please somebody that knows apple to assist.
Actually i suggest you sell it and buy a new PC! Just checked what it has inside wile i was writing this.
Or you want to stick to apple?