I have a GTX 260 in my Phenom II tower, it works well, and both run just fine with my Antec 500W psu.
they may run fine on a 500W....for now
With that video card running as much as 182W and your CPU being either a 95w or 125w, if you put them under max load now you got about 300W for just those 2 alone.
the problem that you don't realize is your running that PSU at close to peak power. PSUs run more efficient and cooler when running at 60-70% of output ability/peak power.
all kinds of factors come into play with PSUs. heat kills any electrical component, the hotter it runs the shorter its life, its just a fact of electrical components.
then there is the aging of the capacitors, PSU lose upto 30% of output ability over time, the average is about 2.5-3 yrs (ranging from 2-5) which means that anytime in that timeframe that 500W will end up being a mere 350W. again the heat comes into play, the hotter you run the PSU (ie.. the more you run at close to peak power) the faster you will reach this point.
you can help this process by running a PSU larger than what you NEED so that the PSU NEVER runs at more than 70% of peak, thus keeping temps a little lower. as well this also will help when the aging comes into play, because you are running a larger PSU than NEEDED, you have enough headroom to safeguard against that aging damaging other components.
my personal rule is (an good advise for anyone) whatever my system NEEDS, I run a PSU that is 30-40% larger and this allows me to achieve, better efficiency, run cooler, and have safety for aging.
So I'm sure your happy with your 500W. However if it is more than a year old I would consider replacing it or you just may end up damaging/frying/losing some components due to lack of power. usually the video card is the first to go because it is quite often the single most power demanding component, follwed by either the CPU or motherboard.