The GS600 is at least decent, yes.
As for it being overkill, take a look at this: http://www.anandtech.com/show/4061/a...eon-hd-6950/24
-- that's with a somewhat more power hungry system since it's built around an overclocked Core i7 920. Your 2500K is less of a power hog. Look at the numbers measured while running Furmark -- 364W at the wall, which translates into ~310W DC. Furmark is kind of an extreme example, so a normal gaming load would be closer to 270-290W depending on the game and stuff. Unless you fold on both your CPU and GPU, this is going to be the maximum average load you'll ever put on that PSU. With that in mind, a good 500W unit would be ample for your system, even with overclocks. And unless you fold 24/7, there's no way your PSU would be at max output all the time.
Look at it this way: if you're like most people, you don't game every minute of your time spent on the computer, and most likely you spend most of that time doing very basic things like browsing the Internet, which require barely more power than idling at the desktop. Meaning you're looking at ~160W average power consumption, spiking to ~280W while gaming, which is <50% of your time, and that's still far from 500W or even 450W.
You can get the GS600, it's a decent PSU, I just wanted to illustrate my point that it's overkill.
Read this for some more numbers: http://www.overclock.net/power-suppl...load-myth.html