Sorry, but for that price range you wont be getting a decent computer that play games. Both the ones you linked can run WoW but it wouldent be very good.
The first one you linked has a ATI Radeon HD 4200, which is integrated graphics. Sadly, it cannot handle much.
The second one I cannot see a graphics card (i hate the new egg site
)
You would be better off getting parts (maybe even second hand) to build a decent computer.
For your price range you can get just about a:
Intel Pentium G620 2.60GHz (You can probually squeeze a i3 on, hardware is expensive in the UK)
ATI HD 7750 1GB GDDR5 Graphics Card
Gigabyte H61MA-D2V Intel H61 (might be able to get a z77 where you are)
Corsair Vengeance Blue Low Profile 4GB (2x2GB) DDR3 1600MHz
BitFenix Merc Alpha Gaming Case
OCZ ZS Series 550W
Western Digital Caviar Blue 500GB (ebay has good prices for HDDs, worth a look)
Samsung DVD RW
This comes a little over your $500 budget when converted from £ to $. Hardware is normally cheaper in America than it is here in the UK so maybe you can get more for your money. Games will run better on this system than the pre builts. Much better
Oh and lastly: don't be afraid of this Pentium! It's a very strong CPU considering its price range. It can handle a lot of games without bottlenecking your GPU (with expection of Skyrim, BF3 etc). Games are not really CPU dependent
I do recommend though grabbing a i3 if you can.