See my sig and listen to everyone else.
We wont let you do that. It can be pretty much as easy as buying what we tell you to. Of course, we'd be happy to explain the reasoning behind what we tell you to buy so you dont just blindly trust us. From there, putting it together is easy as long as you take your time and follow instructions carefully. Then you'll end up with a better computer for the price, as well as a better understanding of the internals of your computer. Easier upgrading and troubleshooting once you know your way around a little, which you gain from just building it once.
So, going off of what you said ($1000-1300 budget):
Sapphire 6950
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102914
Seasonic 850w PSU
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151100
i5 2500k + Asus P8P67 Pro
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.575884
CM 690 II Advanced
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119216
4GB G.skill RAM
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231190
Optical Drive:
http://www.newegg.com/Product/Product.aspx?Item=N82E16827151188
1TB samsung spinpoint F3:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152185&cm_re=samsung_spinpoint_f3-_-22-152-185-_-Product
The only tools you'll need are a screwdriver for motherboard standoffs, and probably an anti-static wrist strap.
Comes to $972.54 with shipping. You can spend more and improve some things, but it's up to you. Improvements would probably be very minor. You could also go with a GTX 570 for $70 more. This gives you a nice upgrade option in the future for gaming by buying another 6950/GTX 570.
Other people will probably suggest different builds or changes. Just make sure you look up benchmarks and do a little research, and ask questions before buying anything.
EDIT: Forgot one of them thar hard drive things (kind of important). Makes the total $70 more.