If you're concerned with the L2 size, the current consensus seems to be 6420 > 6600.
And yes, as you correctly pointed out, I'm guessing the E2140s (1.60GHz) will easily do 100% OC (3.20GHz) due to small L2. Also, keep in mind that Intel can bin speeds, but they can't bin L2. L2 is either pass or fail, instead of A/B/C/D. So even though E2140 might have a small L2, my instinctive guess tells me they will clock like mad. (But don't come back to me with dismal OC. OC'ing voids warranty, you know.

)
I might order one for the hell of 100% OC.
Edit: Quick google search brought up this page.
http://www.xtremesystems.org/forums/showthread.php?t=143549
Please note: These guys' results shouldn't be considered a norm. Not only they exploit 'Xtreme' way of overclocking, but also the e-pen1s competition among them is fierce so they are not always honest with the settings, methods, parts, or even the way they present screenshots, etc. (It's a very general comment. Not towards the specific member who posted her/his results) But at least we can see the performance under a certain clock frequency. It looks like the gap @3.50GHz between E2140 (1MB L2) and E6400 (2MB L2) is smaller than the gap between E6400 (2MB L2) and E6600 (4MB L2). In other words, 4MB>>2MB>1MB. At high frequency (i.e. 3.20GHz and above), the E2140's performance penalty against 2MB L2 CPU doesn't look that big.