My recommendation... Don't waste money on an upgrade
I would build a new PC and try and save up enough money to purchase a system around $1,000 to give you a better all around PC. You can build one for $600 to $700 but will require an upgrade sooner to stay more current with newer games.
You can get an I5-750 with asus p7p55d pro mobo combo for 370.00 new corsair psu 650w for 110.00 4 gigs corsair ram for 118.00 new video card gt9800 nvidia for around 100.00, depending on your preferred brand. new seagate hard drive 7200 rpm 250 gb sata for 45.00. Assuming you can reuse the case, you should only need to spend about 800.00, including a new case for about 55.00. This should be enough to play WOW and the other games you mentioned with no problems. Do some research on it.
This is a pretty good solid case, has a lot of room, and looks pretty good. Good luck whatever you do, but I decided to build new instead of upgrade, only re-using tower case. I am buying my stuff tomorrow.
I haven't read the review in depth, but it uses a few parts that aren't recommended around here a lot.
The G.Skill Ripjaws are recommended a lot, but usually the CAS 7 kit, not the CAS 9 used in the article.
2 video cards isn't usually the way to go at the start of a build, though 2x 4850 is relatively powerful for the price range. One 5770 is more futureproof, with support for DirectX11.
Finally, the hard drive is definitely not recommended. 500 GB platter drives are faster and use less energy. Your options at 500 GB are the Seagate 7200.12 500 GB or the Samsung Spinpoint F3 500 GB, each at $55 with free shipping. These drives are also available at 1 TB (for about $90). Also available at 1 TB is the Western Digital WD1002FAEX 1 TB drive which is SATA 6.0 Gb/s, but it costs about $30 more than the competing 1 TB drives. Don't get the WD1001FALS, this 1 TB drive does not have 500 GB platters.
I had almost the same system as you. 2.2Ghz Dual core AMD, 1Gb ddr2 and onboard graphics. I've added another 3Gb of RAM and a HD 4670 graphics card at december 2009.
My PC rocks again. 2/3Gb RAM costs ~$50 and ~$75 for the graphics card.
I Still find my AMD athlon 64 x2 4200+ (2.2Ghz) fast enough for gaming. As I can play all latest games on mid settings. I think your 2Ghz is fast enough for you too.
Just upgrade your system for ~$100/125, and buy a new pc in around 1/2years. I upgraded too, and I'm really happy with it. Imagine installing all those applications/os'es/games on your new pc (ouch).
PS: That ATI Radeon 4670 is a really energy efficient graphics card. I had a 250W no-name-brand-PSU, and I was wondering if I was able to add this card. And well, it worked! Running a 2.2Ghz dual core/4gb ram/7600rpm hdd/graphics card and two dvd drives on a 250W PSU (lol), while its still stable )
In an effort to see how well a minimal system could run even relatively undemanding games, I paired a HD4670 with a low-power 4850e. If I had nothing else, I could have "suffered" along with it, but the CPU definitely bottlenecked the GPU (at 1440x900). This bottleneck was entirely alleviated by upgrading the CPU to a 720BE at stock.
Your CPU is actually one tier lower than that 4850e. If your mobo is AM2+ (like mine is), you "could" get a better CPU, but then you're also RAM-limited, by DDR2 which is increasing in price and won't be reusable in the future. The games you want to play are also more demanding than mine, so I really think you're looking at a new build. You should have no trouble fitting some decent parts into your budget, like an Athlon II X3, HD5770, 500GB drive, and 4GB of RAM on a 770, 790G, or 890G AM3+ mobo, with an optical drive and a quality 430W-550W PSU (e.g. Antec, Corsair, or Seasonic).