How much better is the phenom II x4 955 than the athlon II x4 635?

Should be around 10-20% depending on what your doing, may actually be lower, not much of a performance difference between Athlon II and Phenom II.

You forgot that the 955 is also 0.3GHz faster clockspeedwise than the 635.

Here's a bench of various tasks:
http://www.anandtech.com/bench/Product/122?vs=88

Here's two processors at identical clockspeeds but one has L3 cache. This is to illustrate the benefit of the L3:
http://www.anandtech.com/bench/Product/105?vs=81

Note that in a lot of tasks they are identical. But in various other tasks, the L3 provides a performance boost. It all depends on what you do with the processor.

Back to the question of value, it depends on what you do. Many tasks will have no noticable performance difference even with the clockspeed boost and addition of L3. On the plus for the cheaper, less powerful Athlon IIx4, it uses less power due to the lack of L3.

Is it worth the extra $60? It all depends on what you are doing.

It's for gaming, most notably WoW.

What video card, settings, etc? If your graphics card can handle a lot like if you have a 5870, the phenom II 955 is likely needed. But less, the performance benefits are debatable. That said, gaming is one of the tasks that makes good use of L3. If it were me, I would get the 955. That said though, I got myself an Athlon IIx4. But take into account that I also have a radeon 3870 and don't intend to upgrade for a while from it.

WoW needs a fast dual core CPU. I've played it on both an Athlon II x3 435 and the Phenom II x4 955. You wont notice a real world difference in your CPU. Save your money for a video card upgrade.