Looking for a good build.

I am looking on www.cyberpowerpc.com to build a new desktop. I am into gaming and want a computer that is built for it. I am though not very knowledgeable on computers. I understand the basics. I am wondering is Nvidia better or ati? would two Radeon HD 4870 PCI-E 16x 512mb DDR5 cards be better then two ATI Radeon HD 3870 X2 PCI-E 16X 1GB Video Cards?
Just looking for some input.
Please comment on cases and cpus and such too. Any information is helpful. Thanks.

 

Indicate how much your willing to spend and you will get some better responses. I believe the 4870 in Crossfire will romp all over the 2 x 3870x2.