Well mainly it depend on the Resolution of your screen, the higher the resolution, the more powerful output of your VGA will be needed. But in my opinion i would say you to use buy an Nvidia GTX 295, install it (PCI-Express Slot Needed)because the graphical output is really worthy. But one thing using an X-Box is that its easy to use, but remember the speed of the mouse acceleration is not available in the Xbox, because using the mouse, the gaming thrill is more since you can control the speed of moving around. Id say you to buy yourself a good Nvidia card because the graphics of the games are the most important.
take care. Also if you can provide me a mail to where you got the X-Box 360 emulator it would be awsome. Thank You