If you live in any of the middle/major cities in the US then use the setup of the month build that matches your price.
Once you've recieved all parts take them to a nearby store where they work with computers/something close to that and pay them like 50-70$ to assemble it, it'll save you alot of money.
Here's something I found on digitalstorms monthly payment:
Digital Storm accepts many methods of payment. We accept: Visa, MasterCard, Discover, American Express, Pay Pal, Wire Transfers, Direct Deposits, and Checks.
Digital Storm currently does not offer financing for our computer systems. If you do with to have financing for your computer system purchase, you can contact your card issuing bank and have them setup a monthly payment agreement or you may setup a loan with your bank and have them mail us a check.
If you have further questions regarding payment options with Digital Storm, please call us: 1-866-81-STORM.
39 bucks a month for 1300 dollar PC? Not to be offensive or condescending in any way, but do you really want to be paying for a computer for several years? That'd just get tiresome for me, personally.
Anyway, OT: Yes, the computer will handle WoW/SC2 easily. The "bad" things you're hearing about Alienware is that they're usually more expensive than other brands/self-building, which is true. Are you sure you can't buy something at a nearby store in a mall? They usually have payment plans.