No problems with power, the PSU has 3 12v rails, each giving 18 Amps, plenty for a GTX260
Before you pay the money, take a look at a few reviews because I think you will be dissapointed with the upgrade. Also bear in mind most review sites use more powerful CPUs than yours so the framerates they give will be higher than those you see, but the hierachy will be the same.
Here is one I prepared earlier:http://www.techpowerup.com/reviews/Galaxy/GeForce_GTS_250_1_GB/1.html
Also check out: X-Bit labs, Hardwarecanucks, Guru3D and Annantech.