well, the first and last on your list are the same machine at the same price, and the middle one has a better graphics chip and comes with fancy speakers, which will make your games run faster with better sound.
only you can decide if it's worth the extra money.
System Manufacturer/Model Number mickey megabyte 1234 OS ultimate 64 sp1 CPU i5 2500K 3.3@4.2GHz Motherboard MSI P67A-GD53 Memory 8 gigs GSkill Ripjaws 1600 Graphics Card amd hd6950 Sound Card creative x-fi gamer Monitor(s) Displays samsung 24" Screen Resolution 1920x1080
Keyboard saitek eclipse ii Mouse logitech g3 PSU antec 550 Case antec three hundred Cooling i'm a cooling fan Hard Drives ocz vertex 2e 60 gig, samsung f3 1tb, buffalo 2tb ext Internet Speed about 4 Mbps Other Info i love win7