Your CPU will become a huge bottleneck for the GTX 260. The GTX 260 needs a CPU to run about 3.6 Ghz to higher to be pushed to its full potential.
You'll see a performance gain by installing a GTX 260 but you should look into overclocking your Q8300.
Can you look at your PSU? There should be a label on it that states max output (watts, amps, etc...)