IMHO, you're asking the wrong question. Nobody should attempt to use a PSU that's just big enough. Using an oversize QUALITY PSU is the cheapest insurance policy you'll ever buy for the continued performance of your new computer. You will probably even be able to use it for your next computer. Stressed PSUs cause all manner of problems impossible to predict/diagnose except by replacing it.
Hm, previously I used a Gainward GTX 275 along with Phenom II X3 720 @ 3.2Ghz powered by an Enermax Modu82+ 425W PSU.. it's got a total of 396W/33A +12V and I could run a lot of games with it..the power seems enough as I never saw my 12V rails go below 11.9V.. 504W with ST65ZF + GTX260? more than capable IMO..
The official Nvidia power recommendation for a pc system with one GeForce GTX 260 video cards is 500 watts or greater with a current on the +12 volt rail rated at 36 amps. The GeForce GTX 260 also requires two 6-pin, 75 watt, PCI-e power connectors.
The power recommendation is the total power for an entire pc system.
The Silverstone Zeus 650 watt power supply is okay but it is a 5 year old model that is the equivalent of a modern, high quality, 500 to 550 watt power supply. At the time the model was introduced it was considered to be a very good psu. back then there were no certifications for energy efficiency. By today's standard the psu is not energy efficient.
The maximum power on the +12 volt rail(s) is 504 watts. To determine the current (amps) divide the 504 watts by 12 volts. That works out to 42 amps which is weak for what is supposed to be a 650 watt power supply. A current of 42 watts is what one expect to find in a modern, high quality, 500 to 550 watt power supply.
Despite the older technology and lower current it should still be okay.
Do you already have the power supply? If not, you should consider a modern, high quality power supply.
Corsair and Seasonic are two brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty in the USA. Some of the newer models come with a 7 year warranty. Lately we've been seeing a few other brands offering some high quality units. One example would be the Antec Earthwatts and Antec TruePower New models which are a major improvement over Antec’s older psu’s like the Basiq models.