Mine is 2.4 GHz, which is what Apple considers the bare minimum for unassisted 1080p H.264 playback for iTunes content.
http://support.apple.com/kb/HT3209
Note though that the 2008 white MacBook also came in a 2.1 GHz version, so if you had that one you'd be SOL for 1080p H.264.
I do agree that even with a 2.4 GHz though, YouTube H.264 can sometimes lag more than iTunes, but I find that if I launch a 1080p HD video in YouTube and then leave it alone, it will usually play cleanly after a few seconds of settling in.
That said, for YouTube I just stick to 720p (or less) because the screen is only 1280x800 anyway, and 720p stresses out the computer less.
The above specs are completely irrelevant for the 2009 MBP though, as previously mentioned, because of the hardware H.264 assist it has.
OK that makes sense. Your Safari usage alone probably eats up a few GBs. However, I wonder what the benefit of manually purging memory is, vs. just closing applications when not in use.