The lowest spec machine I have been able to get 60fps 720p video out of Youtube is a 2nd Gen i5 2.4Ghz. Anything lower and the frames start dropping.

It is funny that during that time period, Intel's media decoders got significantly faster in a very short period of time. My 4th gen 2Ghz i5 decodes video better than my 2nd Gen 3.8Ghz i7.