What power supply do you have now?
The recommended power supply would be 500W and have +12 Volt current rating of 36 Amps.
Thats from an EVGA recommendation for that type of GTX 260 video card. http://www.evga.com/products/moreInfo.asp?pn=896-P3-1255-AR&family=GeForce%20200%20Series%20Family
An example of a PSU that meets those requirements is Antec EarthWatts EA-500D
which has 37Amps of +12V.