At 4.8GHz the FX-4100 should outperform an i5-2400.
Generally speaking the Phenom II, FX and Core 2 Duo / Quad basically perform similarly at the same clock speed in games and some applications that do not use more than 4 core very well. On average Sandy Bridge CPUs perform around 22% better than those processors.
Therefore, the FX-4100 only need to clocked to about 3.8GHz to have similar performance to an i5-2400. Note, this is general performance specific performance regarding a particular program or game can vary.
The FX series has a lower IPC than the Phenom II (which was about equal to Conroe's as you said). Your figure is way low if you look at benchmarks. People have tested overclocked FX-4100s that can't stand with the i3-2100 in games (with the same 3.1 GHz clock) and the i3 is a dual core hyperthreaded chip.