Hmmm...As far as I know, in X the only way to actually free up RAM is to quit an application. It seems that some here are suggesting that Windows behaves differently. Is that the case?
I've got an i7 27" iMac running 10.6.8, with 12GB RAM, and it seems zippy enough for me. However, I only do basic adjustments with LR4. No PS work with layers and all that, nor HDR. And I usually run no other apps when doing PP.
As Lion doesn't bring anything to the table I'm interested in, performance or bells and whistle wise, will be staying with 10.6 until new hardware forces my hand. But, if already moved to Lion, it would probably be a good idea to upgrade to ML. 10.6 saw real improvement over 10.5.
Maybe folks should step back a bit and remember what it was like to do this stuff just ten years ago. We would have been quite jealous of even the mediocre equipment available today.