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.
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.