to get hardware acceleration for youtube flash videos you need at least a geforce 8 series or at least a Radeon HD 4000+ card
however it is very difficult to find a geforce 8 AGP, they are all PCI-E
also for old computers is better and faster to use flash player 10 instead 11
flash player 11 is slower on old computers
my backup computer is a pentium 4 cedar ii at 3.0ghz and a geforce 6600GT, it can play youtube videos up to 720 very well using flash player 10, in 1080 not works fine skip lot of frames cpu is at 100%
ie note this config uses CPU only because the geforce 6 series not support hardware accel in flash player
