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.