The Core i3-2100 can be significantly faster than the PII X4 955 in games that only require two (and probably 3) cores. Most upcoming games will still only use dual core CPUs. As for games that can use 4 cores, the i3-2100 will be marginally slower. See following for some benchmarks.
Of the 6 games benchmarked, the i3-2100 is only 0.9 frame slower than the PII X4 955 in Mafia II. But the in Far Cry 2, the i3-2100 is faster than the PII X4 955. Those are the only games benchmarked that uses quad cores (that I know of)
Dragon Age: Origins has been coded to make use of more than 2 cores, but the i3-2100 provides better performance than the PII X4 970.