A few things to remember...
Keep in mind that although games usually use two cores, many games are being optimized for four and, with the upcoming release of the PlayStation 4 and the next Xbox, even more cores could be used.
When it comes to AM3+ motherboards, you should use a 900 series chipset as it has all the enhancements for the FX line of processors whereas the lower chipsets offer physical compatibility with the FX CPU's and not much else. A typical 970-based motherboard would be a good choice. The 990FX chipset is intended for people who are using Crossfire or SLI.
With FX CPU's, you should use Windows 8 as it has the scheduler fix "baked in" whereas 7 has it as a patch. Having this optimization fully in place rather than patched into the OS does yield some slight performance increases.
You also don't need a network adapter as it is already on the motherboard. The only exception is if you are going to be using WiFi with your desktop PC instead of plugging it directly into your router.
Your power supply capacity is determined largely by the graphics card and 550W should be plenty for single card configurations. As for which GPU to get, that depends on the games you play and your desired performance. For basic performance, I would go for the Radeon 7750, which is roughly equal to the GT 640, or the Radeon 7770 which is around the GTX 650.