A mainboard with integrated graphics is not what you want to buy. Integrated graphics are fine for anything but gaming, hence the board may not be intended and designed to be the heart of a game rig.

You chose a graphics card that is old. There are 4850 and 4870 out, and even 4870x2. Usually that means you get more hardware for the same money and the old cards are marked down in price.

I'd go with 8GB memory. Make sure you don't get fancy modules with cooling stuff that avoids that you can plug them next to each other. 4x2GB does the trick and keeps you using dual channel while using modules that are mass market priced.

Make sure that when you buy all that stuff, you do not forget an adequate processor cooler. You need a serious heatsink and fan or equivalent cooling output.

I don't know how much juice the AMD needs, but you need to look carefully at the graphics card(s). Depending on your choices, 500W may not be enough.