You've got a very, very good chance of hitting 2.4Ghz + as in you're almost certain to get it. After that it depends.
1) Your CPU overclocks on the stock voltage to 2.6Ghz, you leave it there and be happy
2) Your CPU overclocks to 2.4Ghz and no higher so you increase the vCore to 1.45 and you increase the speed to 2.6/2.7Ghz.
3) Your CPU hits a brick wall at 2.5Ghz and never goes faster.
Or some other combination of possiblities.
Now, how to overclock: Mostly it's done in the BIOS, there should be a guide in here somewhere about how to do it with an A64 chip, read it, follow it, if it doesn't work then put up a thread saying what isn't happening and ask why. There are things like the AI booster for ASUS boards, but they're a bit crap so i wouldn't bother.