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As kitamesume pointed out, this tech forum has several excellent topics on building systems. There is a lot of detailed discussion about parts and their capabilities ... discussions that would help you very much in evaluating pre-built systems, also.
So, urgently advise you to use the Search function. If you click on it without entering anything, you will be asked "
Did you forget to put something to search for?" and offered the
option for Advanced Search, which is powerful. You can search only the Tech forum, for example.
Beyond that, I will always recommend that people build their own desktop. You get the best value for your money, and what you learn by doing it will make you a very literate user, someone who is in control of their system and won't have to helplessly take it in for simple problems and get gouged on service fees.
That's your decision. Building is not horribly complicated so long as you learn basics and are careful & methodical; there is plenty of help here to get you pointed in the right directions, and you can search youtube for videos that will show & explain in detail everything you'll need to know.
So first questions: is your budget for the computer "box" only, or does it need to include the monitor, keyboard & Mouse?
$1000 is very, very generous to build a fairly l337s rig for most gaming if it's for the box only. If you need to reserve $250 or $350 for a fairly high-end monitor, then the budget starts to get tight, but you can still do a hell of a lot for $700 or $800 on the box.
Consider that you will start off with 8 GB of decent RAM. You will not need more to start, and you can budget an upgrade for "down the road" if/when it becomes critical.
Alienware is crap for the money you spend on it. Don't get sucked in by their marketing; it's designed to separate you from your money, not provide you with good performance vs. value. Even if you go pre-built, you can get a
LOT better value for your money.
Nvidia does not suck. Anyone who tells you that is a fool that you should avoid taking advice from. There are serious advantages to their products depending on what you will use them for and what your budget can handle. There are also advantages to going with other options. Keep an open mind at this point.
Since you have plenty of mass storage capacity, one of the things that will be serious consideration is to look at Solid State Drives (SSDs). Right now, 128 GB SSDs are of exceptional value for the useful size and performance; with care you can get an outstanding one for ~$100 delivered (
Samsung 830 series or
Crucial M4 or
Mushkin Enhanced Chronos Deluxe are some of the better, although not only, options ... learn to watch for them on special and grab them fast when they turn up on one).
Are you in the U.S. ? Can you use newegg.com within the U.S. ? If not, what region will you have to purchase in? Subscribe to the newegg E-BLAST newsletter (homepage on the right edge, about halfway down the page). You'll get notices about five times a week or so with all their current deals, and you can save a hell of a lot of money by watching them.
List a few of the exact, highest end games you wish to play, also. That will give a solid idea of how much power you actually need.
But, definitely take some time to read through a few of the more recent topics here. It'll help us out a lot if you increase your basic literacy on the tech.
Then, think carefully if you want to build or buy a prebuilt and we''l take it from there.