What graphic card should i buy?

lol what I notice is that your CPU completely blows away all your other computer components. I would just honestly go with the GTX 580 because the extra premium for a 590 is not worth it. I would rather spend the extra cash on an SSD, which will give considerable performance than purchasing the GTX 590. Also, the GTX 580 alone completely blows any game out there in the market. Since you'll have like a little over $500 left, you'll notice better performance by swapping out your hard drive with say a OCZ vertex 3 and get a bigger monitor. In my opinion, a 23" monitor is a bit small for my taste especially when gaming. I'm sure you'll be a 'lot more happier with a 27-30" monitor.