which is the best Dual Core or Core 2 Duo
a little subjective, the "best" dual core is the E5300; mine overclocked to 3.9Ghz without breaking a sweat, leaving the higher 'stock clocked' E6800@3.33Ghz in the dust.
C2D? again subjective. though the E8600 has a higher stock clock @3.33 it didn't seem to fair as well as the E8500 that would hit 3.9Ghz with just raising the fsb to 1600.
i3? i gotta be *very* biased and say the 2120, best bang for your buck sandy bridge dual core.