Honestly, after looking at benchmarks, I think I'm feeling the i3 2100 over the AMD 955. The 955 will do a bit better with quad-core enabled games when overclocked...but at stock speeds, it very barely edges out the 2100 (by maybe 5%). When it comes to games that only support 1-2 threads (like Starcraft 2 or emulators), the 2100 will be significantly better. They would run about the same price, but going with the 1155 motherboard leaves a much better upgrade path.
Thoughts?
edit: actually, it probably beats the stock clocked 955 even in quad-core games, judging by the benchmarks here.