Well if you just want the monitor, you can buy a minidisplay -> vga/dvi adapter for your macbook and set it up like a desktop.
http://www.cellphoneshop.net/midipotovgaa.html?cagpspn=pla&gclid=CNjX0JGx1LQCFSWoPAodd3QAeg
I have played wow on laptops for the last 4 years because of college. I have concluded that I may as well just buy a desktop because when I'm playing wow it is set up as a desktop. Most recently I was playing on the 13" macbook pro set up with the minidisplay to vga for a monitor with keyboard and mouse. This was the same thing as a desktop. So after I spilled wine on my laptop, I built my first desktop for around $900, and it works beautifully on ultra settings in wow with 60+ FPS. It was less than my macbook pro and at least 10x better.
So are you getting a macbook and a desktop or just one? After owning a macbook pro, I don't think I'll ever buy another mac. They work fine under normal conditions, but I find windows much more intuitive and easier to work with. Anyway if I were to buy another laptop, I'd get the PC ultrabooks (Samsung makes a nice one for less than half the price of a mac at about $500) then have a desktop for the gaming.
But to finally answer your question, check out newegg. There are a ton of prebuilt computers. I would recommend building your own if you haven't. It is surprisingly easy and a lot of fun. You can customize exactly which components you get, and you can make a reeeeeeeeeeeally nice one for $1000-$1500. That being said, it may not be cheaper than buying a prebuilt, just customized and a fun experience. If you don't want to build your own, just look for a computer that has what you want. For that price range, here is a list of some things you would want to look for:
CPU: Ivy Bridge I7
Memory: 16 GB RAM & 500+ GB ROM & 128+ GB SSD
Video Card: Radeon HD 7950+ or GeForce GTX 670+
These three components should be your most expensive and crucial parts. When trying to select which components you want, tom's hardware is an excellent resouce:
http://www.tomshardware.com/ . They have a ton of information on which components are the best in different price ranges.
If you want, I can email you the computer I made, which would be slightly less than your price range. Hope this helps.