At the heart of every good gaming system is a good graphics card.
This 3D powerhouse generates the images you see on the screen. In basic terms a 3D game holds the information required to generate a 3D world and it's the graphics card that takes this information and turns it into an on-screen image.
Two companies make the only gaming cards worth considering: the AMD Radeon and Nvidia GeForce cards. Currently AMD has its HD5000 range and Nvidia its GTX 200 and 400 ranges.
The short rule is the more you spend the higher the resolution, the more effects you can use and importantly, the smoother the latest games will play.