Shitty world.com is shitty. Do you know how much they put on there prices that you don't need to pay? Value desktops is what you are looking for, but not from there.
The specs that I'd recommend are either a nice intel dual i3 @ a clock speed of 3.3 Ghz or even a AMD Quad-Core for abit more punch if you want to render videos or speed up multitasking.
As for RAM and OS, windows 7 doesn't really require anything over 4, so you'll be better off going with Windows 7 32-bit, but if you want to, you can probably get 8 GB's (So that would be a 4 x 2 GB Ram modules) Although that would require a 64-bit version of windows which maybe be more expensive.
In terms of the graphic card, for the budget you are on, it is probably not worth spending it on a system like Xfire or SLI (Ati's and nVidia's dual and beyond graphic card sytems respectfully.) I'd recommend a 6850 or 6870 due to if you want to play some more modern games - but this does mean a big portion of your budget is being spent on the gaming section.
Its always benefical to build your own as well! You can generally save abit, and you'll learn much more about the inside of the computer. Come see me and Fissurez in IRC too!