Which computer should I get

Not a single one of those is worth the cost.

Building your own is cheaper, allows you many more options for customization, and also can be focused onto a certain task (like gaming).

For the amount of money you are spending on these pre-built computers, you can build a much better "gaming" computer

Pretty abysmal gaming configurations. i7-2600 CPU w/ GT 530 / GTX 555Ti / HD 6750 graphics
What resolution is your monitor?

Of the 3 you listed the one that is the least abysmal is probably the Gateway because it's $100 less than the SYX.

Can you find an option that gives you a Core i5-2500 and Radeon HD 6850?

Would this be better than the 3 I listed? http://www.cyberpowerpc.com/system/Gamer_Xtreme_2000/

If not, how do I build my own and will it really be that much cheaper?