They don't do well in single threaded games such as Skyrim or Starcraft 2. However, they can hold their own. Most of the work comes from the GPU anyways.
The Phenom II's are actually still available on some websites. Here's a trustworthy site...
AMD Phenom II X4 965http://us.ncix.com/products/?sku=46281&vpn=HDZ965FBGMBOX&manufacture=AMD&promoid=1033
I would highly suggest it over the FX 4100. Do you already have a motherboard? If so, can you give me model number or link me to it?