|Do you have Vista Home Premium 64-bit or 32-bit?|
You definitely have enough power in the PSU to run it, and your motherboard can handle a GTX series graphics card.
BTW, if you didn't already know, the GTX series graphics cards take up 2 slots on the motherboard and are about 10.5" long. Because they take up 2 slots, you won't be able to use that PCIe x1 slot to the right of the graphics card slot (PCIe x16). It will be covered up by the graphics card, so hopefully you have no need for 2 PCIe x1 slots. I don't think length should be a problem at all (~10.5").
Also, an ATI graphics card offers about the same performance as an nVIDIA graphics card at a fraction of the price, so you may want to think about that.
I think a good alternative would be the Radeon 4870. It is about equal with the GTX 260 in terms of performance, but also cheaper. How about this one from Newegg?: