I just tried that.... no issue. Doesn't make sense but I think you'll have to full wipe.So I was on 1.6.1, installed 1.6.5 successfully (thanks Sho!). Phone booted up after the install. Played with it a little bit and then went to Task Manager, as soon as I pressed Clear Memory the phone re-booted and never started again. I am able to go to CWM, I re-flashed 1.6.1 but it still won't boot, it is stuck on the Samsung Galaxy SII logo. I'll try a restore.
Oddly enough why do you feel the need to clear the memory. All it does is make apps start slower which in turn makes the phone feel sluggish and in turn usually uses more battery.
Hope you get it running quickly.
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