My cousin has decided to build the best possible desktop that he can for $1200-1400 but both me an him are noobs when it comes to computers.

First off why are AMD's multi core processor's so much cheaper then the i7 core processor's? For instance I can buy a AMD Phenom x6 processore for $190 or a i7 quad core for around $230.

And we want a top of the line graphics card that will run The Witcher 2 pretty much as good as possible.

So I could use your input on what components we should get to make this a top of the line gaming computer.

Man a lot of people are building PCs at the moment 

the i7 as long as it's sandy bridge will be much better for games than the AMD

I am not sure any reasonable graphics card can run the Witcher 2 "pretty much as good as possible" ATM "Ubersampling" is a killer but the game looks great on even a mid range card. A GTX 560 TI (make sure it's the Ti version) is probably the sweet spot at the moment for price/performance. 

Other than that some more info like if he has or needs a case, mouse and keyboard, monitor, OS etc as that will affect what he can afford.

There are two reasons why AMD CPUs are much cheaper especially at the high end...

1 - Intels high end processors are better than AMDs high end processors

2 - Intel charges way too much for their products (bear in mind I have no idea how much it costs to actually produce a single processor)

That being said the best available AMD CPU is really enough for the most demanding games currently, you will find the the I7 range of CPUs really excells at things like 3d rendering in things like Maya and ZBrush and profession programes such as Photoshop and the like.

As far as a GPU is concerned AMD is much more competative and in many ways actually better, particularly with DX11 so a card like this http://www.newegg.com/Product/Product.aspx?Item=N82E16814127581 is a pretty good option.

Intel processors work a lot differently to AMD processors, it isn't always about the Ghz or the cores, the best Intel processors tend to be better than the best AMD processors.

I would say that Intel are slightly more expensive for the performance you get, you can compare processor benchmarks here http://www.cpubenchmark.net/.

With that budget, I wouldn't be afraid to splash out on a high end Intel i7.

The graphics card mainly depends on the type of game you want to run and at what resolution.

The Witcher 2 maxed out @ 1080p would need a GTX 570 or an ATI HD 6970 for it to run smoothly.

If you have a lower resolution monitor or you do not want maximum settings then a GTX 560 or an ATI HD 6950 would be fine.

Again, with that budget, I would recommend you go for the high end graphics cards.

He will need a case (but those are about $50) and the OS system, we have mouse's and keyboards and he's thinking about getting a decent monitor for the game. Will plugging the desktop in via HDMI to a HDTV look good?

We'll probably get the GTX 570 like someone below mentioned, and get a good i7 processor....BTW does the more memory a graphics card have = it being better?

Now what kind of motherboard would work with the above? 

And what do we need for a power supply and fans and all that stuff?

Built a semi-beast rig last year around this time for work stuff (Excel megaprocessing with ~100MB files and billions of lookups). Good stuff. $1500 bought me just about everything imaginable without going too overboard.

Highly suggest Intel i7's. Awesome processor. Huge increase in computing power for my work (8 threads on excel is wonderful). I am unsure where you live in America, but try to locate a Micro Center. If you have one nearby, you can save hundreds on a top-of-the-line processor. I paid $200 for a $300 processor.

Tomshardware.com has the best forums for new builds. For $1,500 they suggest (as per one build):

CPU: Intel Core i5-2500K Sandy Bridge 3.3GHz $224.99

Motherboard: ASUS P8P67 PRO (REV 3.0) LGA 1155 Intel P67 SATA 6Gb/s USB 3.0 ATX Intel Motherboard $189.99

Hard Drive: AMSUNG Spinpoint F3 1TB 3.5" SATA 3.0Gb/s Internal Hard Drive -Bare Drive $62.35

Power Supply: CORSAIR HX Series CMPSU-850HX 850W ATX12V 2.3 / EPS12V 2.91 80 PLUS SILVER Certified Modular Active PFC Power Supply $164.99

DVD Drive: Asus 24x DVD±RW $21.99

OS: Microsoft Windows 7 Home Premium SP1 64-bit English 1-Pack - OEM $99.99

GPU: SAPPHIRE 100312SR Radeon HD 6950 2GB 256-bit GDDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card with Eyefinity $274.99

GPU2: SAPPHIRE 100312SR Radeon HD 6950 2GB 256-bit GDDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card with Eyefinity $274.99

Case: Cooler Master Storm Enforcer USB 3.0 Mid Tower ATX Case with Window and Black Interior (SGC-1000-KWN1) $85.97

Cooler: COOLER MASTER Hyper 212 Plus $39.99
Reply to swh127

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Only thing I would tweak would be an SSD like this one (40GB): http://www.newegg.com/Product/Product.aspx?Item=N82E16820167044

If I was buisling a mega super awesome PC for that much money, I'd get something like this.

CPU - Intel i5 2500k (don't even look at anything else) ~$200

Mobo - SLI/Crossfire capable mobo made by Asus, MSI or Gigabyte ~$140

GPU - 6970 or GTX570. ~$320

RAM - 8 GB (2X4GB) tripple channel DDR3 ~$100

Case - Full tower case with lots of fans ~$120

HDD - 120 GB SATA 3 SSD 1 TB 7200 RPM hard drive ~$200 ~$60

Power Supply - 800 Watt Antec (or similar quality brand) ~$130

DVD DL Writer with Bluray OR DVD DL Write BLuray ROM Drive ~$100

CPU heatsink - Noctua NH-D14 ~90 bucks

I'll actually be building a monster PC myself in about 3 months. My Phenom II 940 CPU and mobo architecture is sturting to hold back my frame rate and I am changing to Intel camp.

^^Yeah there's a microcenter 20 miles away so we will probably go there to get everything. I can get a i7 core at microcenter for $225 so I'll skip the i5

Err...which i7 is $225?

Sandy bridge i5 2500k usually beats any previous i7 except the brand new Sandy bridge one (2600k) ... when it comes to gaming at least. Just cause its i7 doesn't mean its best. You have to look at which chipset its using.  1155 is the one you want.

Here is a nice comparison. i5 even beats the new i7 on some games.

http://www.anandtech.com/show/4083/the-sandy-bridge-review-intel-core-i7-2600k-i5-2500k-core-i3-2100-tested/20

I have heard that you may need to tweak some things to get text to look right on HD TVs but that is pretty simple. 

Memory size isn't a really big fator 1GB is more than enough for 1080p with AA you only really need more than 1GB if you are doing crazy high resolution monitors.