The 2.0a is a risky overclocker, that is say, it is unacceptably possible that you may end up with a FSB less than the now common 133MHz (533MHz quad-pumped) FSB. I would choose the 1.8A over the 2.0a as it obtains a fast and stable overclock of 18x133MHz = 2.4GHz. Which is exactly the same as the 2.4b. Thus my recommendation is neither the 2.0a nor 2.4b, but rather the 1.8A. Save your $$$ and improve other components of your system.
<i>It's your world kid!!!</i>