Re: New to graphic cards, have a question about upgrading to 260.
Off of Gigabyte's website: "Superb bandwidth AGP 8X graphics interface"
Right there tells you it won't work with the GTX260. GTX series is PCI Express, the X800 you have atm is AGP, and no they won't work together.
If you really want to get the 260, then you'll need at minimum a motherboard upgrade. If you're going to do that, you may as well pick up a faster CPU and some faster RAM as well.