First off, I'd like to start with a little story.
My friends old computer was loaded with viruses and bad stuffs, so he bought a new computer. He bought a Dell. And "Dude he spent a lot"(pun intended). He spend nearly 6 grand on a computer..and the specs on it weren't even that great. (Definitely not worth 6 grand). I offered to build him a computer, a third of the price, twice as powerful. And I SO did it. Spent 2.3 on an amazing computer.
I would ALWAYS recommend buying custom. Even if you don't know how to build a computer, you can pay someone to do it WAY cheaper then it would be to buy a brand name one. I would always go with
Newegg. They have the best prices I've seen, and all there products have customer reviews so you can really get a feel for what it can do. A lot of there products offer FREE shipping! If not, you'll never pay more then 15 dollars for shipping the biggest of computer parts..