CPU: Intel I5-650
Motherboard: Stock motherboard from Dell Studio XPS 8100 (motherboard and CPU are the only things left from the original computer)
OS: Windows 7 Home Premium
HDD: Western Digital 500gb 7200rpm/ Western Digital 1000gb 7200rpm
Graphics Card: ASUS EAH6850 (Radeon HD 6850)
Monitor: ASUS VS248 24"
Keyboard/Mouse: Corsair K90/M90
Your PC looks pretty good. I would get a 120 GB SSD, enough for Windows and some games. SSDs speed up install times, map loading times, and also help load textures and meshes quicker (ever seen grass grow before your eyes on BF3? I do, even with a 7870. It's my cruddy mechanical hard drive).
Wait until the Radeon 8000s to upgrade your GPU. 6850 is good for now. It can handle all current games on respectable settings.