I know these guys are saying to buy something from Newegg, and put it together yourself. Now, if you have no clue how to do that, I would strongly recommend that you buy an "off the shelf" system. There are tons of gaming PCs out there. Will you spend more money on them vs. building it yourself? The answer is YES, especially if you can recycle some of the parts you already have into the new PC.
Put it to you this way. I reused my hard drives, optical drives, case, and power supply from my old rig (built 3.5 years ago) and spent $800 on a new CPU (Intel Core i7-860 Lynnfield 2.8GHz), CPU cooler (COOLER MASTER Hyper 212 Plus), RAM (G.SKILL Trident 4GB), motherboard (MSI P55-GD85), and video card (XFX Radeon HD 5770 1GB). This think smokes WoW on ultra at 1680x1050. In some areas I get way over 200FPS. If you want to spend extra money anywhere, I'd recommend you put it into the video card, and hard drive (that is, if you use similar parts to mine). My next upgrade is the hard drive. i have 2x WD Raptor 150gb in a RAID 0, and I want to replace them with a solid state drive. Upgrading the parts I have with a ~$300 or more video card, and a SSD would make this rig absolutely smoke.
Good luck, whichever way you go. Don't be intimidated by building a new machine. Just do some reading, and find a guide that takes you step-by-step on how to put together the machine and once you know how, you'll never have to buy another off-the-shelf PC again. I like building mine, because I can build a machine with no compromise. I can pick the best parts, and put them together. Something you can't do with anything probably except for the very high-end machines (Alienware).