600w should be enough.
On full system load your cpu will draw around 100-150w depending on clockspeed, your gfx around 250w and the rest of your system ~100w. That's around 500w in total while your pc is a maximum load. You will not reach anywhere near this load unless you run extreme benchmarks on both your cpu and gpu at the same time. Playing games does not draw this much power.
If you wanna be _really_ safe then buy a 700w. Anything above that is just a waste of money unless you plan on buying 2 gfx cards.
My system consists of a i7 2600K, 2x GTX580 SLI and 8GB ram. Runs plenty fine of a 1000w psu. The psu doesn't even get warm.
@Mihir: The main difference between 2500k and 2600k is hyperthreading. That doesn't help you in wow. There is a slight difference in clock speeds, but not worth adding over 100 euro just for that imo. You can easily overclock the cpu anyways as they tend to be able to run on just the same frequencies. SSD sure is nice, if you have the money for it then add a 120gb ocz vertex 3 or something like that. It's not gonna ruin your experience in any way if you don't buy one though. Just speeds up the loading times in wow a notch.
Btw gfx cards is VERY important for running wow these days. Since they added the dx11 features the need for a good gfx card is just as important as cpu. Shadows and grass takes a major toll on the gpu. I did some testing in Nagrand when I got my new pc. I put everything on ultra and found a spot where I had exactly 60 fps (with one gfx). Then I enabled SLI and logged in at the exact same place, BAM 120 fps. Even then I fear it was still my gfx that was the bottleneck
