My GTX8800 plugged into the available PCIe slot, but I had to by a new PS and the GTX8800 needed more power than the PCIe slot could provide so the GTX8800 came with two seperate power cables that plugged directly from the GTX8800 into the cable assembly on the power supply.
Below are the power requirements for the GTX260 that you are looking at. The below came from
http://www.newegg.com/Product/Product.a ... 6814130370 System Requirements Minimum of a 500 Watt power supply.
(Minimum recommended power supply with +12 Volt current rating of 36 Amp Amps.)
Two available 6-pin Molex hard drive power dongles
You can see that it states 500 Watt PS minimum, so you need a new PS. If you have the money I would get one that is more than 500 watts, it is better to get a little more than you need, besides you may want to add something else in the future like another hard drive.
You can also see that it states: Two available 6-pin Molex hard drive power dongles
That means that you will probably need to plug those into the GTX260 and into the power supply cable harness because the PCIe slot can not provide enough power.
I do not know about the GTX260, but the instructions that came with my GTX8800 had no directions on how to hook these extra power cables up and it took me two or three tries to get them plugged in the way that the card liked them plugged in. If they were not plugged in right then either the GTX8800 or the PS was emitting a high pitched whine.
Rod