It could be your internet connection.
Just helping my parents sell their iMac G5 2.0ghz with new 500gb hd and 2 gigs of ram and noticed, almost horrifyingly, that since they recently upgraded their internet to digital, that their computer is fast as lightning.
(And just had them get a new Mac! They didn't really need one -- it was the internet connection! They had cable internet before this...)
Don't feel bad, my Mac mini 2.26 with five gigs of ram could not play that video in HD. I've got DSL. Bet is the network, not our computers.