Cata has up'ed the playig field once again theres even a DX11 mode.
I actually play Wow on systems just like the OP is talking about. My laptop has a faster processor but still uses the GMA4500 video and wow is definitely palyable on low settings. Like wise, the pc I have for my grandson is a Compaq with an AthlonII x2 240 with integrated 4200 series GPU and it can play wow at a higher detail level but setting it down to low still helps framerates. You will still have graphics loading delays when you enter populated areas but it should be better with the 4gb of ram.
BTW - if you assign 1gb of ram to the integrated 4200 then you will only have 3 to game with. If you , down the road, want to add a dedicated GPU then you will most likely need to upgrade the Power Supply as the Dell comes with something like a 230w which is not enough to power even a medium power GPU. I have since put in a Radeon 5670 in my grandsons pc so he could play some games that would not run on the 4200. At the time the 5670 was the highest card I could add without changing the PSU but now there is a 6670 should you come into more funds down the road. He has a 250W psu in the compaq.