What Would You Rate This Computer For Middle-High End Gaming?

You should consider buying an AMD dual-core or quad-core processor as games only use up to four processing cores (and most modern games will only utilize two or three cores). I would not suggest a Phenom II X6 or an Intel Core i7 processor because both are overkill for gaming. I would suggest buying a Phenom II X4 955 Black Edition which is much cheaper than the Phenom II X6 1090T. You can also overclock the Phenom II 955 Black Edition more easily than AMD processors that are not Black Edition chips because it has an unlocked clock multiplier. With the money you save, you can buy a faster graphics card, such as the AMD Radeon HD 5870 or NVIDIA GeForce GTX 570.