Woah calm down there >.> If you want to play games isn't defiently not apple. The other companies don't really matter like Rale_King said the spec count.
Here is a website where if you got a lot of money you could go get a really goodish computer. Or this one.
Otherwise look for computer with at least 512mb dedicated video cards. With 4-8 gigs of ram. Maybe an i7 processor and a 1TB hard drive
Seconded! Although if your going for gaming. You want a good graphics card w/ 512mb vRAM (atleast) DDR3, an i7 processor, any processor that's i7 should be good enough speed wise, atleast 4gb RAM. You could probably get a decent dell/hp for $800 (computer only). Alienware is a huge waste of money. Unless your rich/spoiled that is...
The best gaming PC will be custom built ones. I am with xXforforXx on this one. It will probably cost you a bit more upfront, but it will last you longer. At least my custom built one did.
