It depends almost entirely on how lucky you are with the chip but on stock most e2140's will get to 2.6, alot will get to 2.8 and some will get to 3.0 GHz ... though you need to keep an eye on temperatures as you find out the max you can get.
With decent air cooling ... most = 2.8-3.0, some 3.2 and higher, a few 3.5.
Once you start getting to the higher clocks though, it comes down to the motherboard quite a bit. Also the BSEL mod can help as some of the e2xxx series hit a low FSB cieling which the BSEL mod helps remove.
ok... so..... calculating - assume 3.0 ghz.... then about 20% clock for clock due to architecture..... that would be..... 150% of my x2 4200? (which is OC'd.... but not much - my mobo is **** - another reason to go with this... )