My suggestion: Give up now.
Sorry to say, but what you're asking is like trying to get 5 star food at fast food prices.
You MIGHT be able to build a system that meets the minimum specs for some newer games for that kind of money, but we're talking along the lines of it runs... At a very low resolution with all the detail levels set as low as possible. Even with some masterful exploitation of Black Friday deals, for under a grand you're just not going to really be able to do it.
A possibility you may not have considered, and of course it depends on the MMORPGs in question, is a game console instead of PC. Call of Duty 5 should land on both the Xbox 360 and PlayStation 3, and some other games may as well. I don't know what other games you are interested in, but a console is about the only way you can do it on the budget you have.
Now, not to be all doom and gloom, what you want to do is build this system from parts yourself. Not sure if you'll be able to get it all done by Christmas, but if you start researching now, and really work hard at it, you might just be able to squeak by.
Start hitting hardware review sites and look for any sort of "best bang for the buck" type reviews they have of multiple models of a given component. The idea is to build a system with maximum upgrade potential, so even if the initial specs aren't that great, you should be able to swap in better parts as your budget permits. You want a motherboard with as many memory banks as possible, as many PCI-E and PCI slots as you can get, SLI/Crossfire ready, able to accept multi-core graphics cards, able to accept DDR2 and DDR3 RAM, and ideally able to accept quad core CPUs with a clock speed at least 500MHz/core faster than whatever you plan to put in, but more is better. You also want a case with lots of extra drive bays, a quality power supply with ample juice to carry you through a couple of video card upgrades. RAM is cheap, so buy loads of it. Don't worry about dual channel crap, it's really not going to net you much gain. Better to have twice as much RAM running a little slower compared to half the RAM running a little faster. More RAM means more data can be cached in RAM, meaning fewer trips to the very slow hard drive.
Like I said, the name of the game is expandability. You want as much future expansion potential as possible, so even if your initial specs aren't so great, you can gradually replace parts over time.
To do this well, it takes a lot of work researching parts and picking out the best ones for the money. You will probably spend more time researching parts than actually building the system. The result though, is often a system that is rock solid, and can be easily upgraded for several years, to the point where if you keep all the spare parts from the upgrades, you can eventually reconstruct the original system.
I am looking to build a computer with my dad as my christmas present. We are tyring to build a good gaming computer for under $1000.00. I would like the computer to be able to run games like Call of Duty 5 and a few other mmorpgs as well. If anyone has any suggestions for us, they would be very appreciated.