Powerfull processors are one thing but if you're running a slightly older system like I was (up to 2 weeks ago) DXVA (that's Direct X Video Acceleration) can help.

The real trick is finding a media player that supports dxva with the necessary codecs built in. Sadly the only one I know of that requires ati radeon hd or nvidia 8/9000 series graphics accelerator or better (but at least it's free software) -

http://mpc-hc.sourceforge.net/

It does however have a native 64bit client for you 64bit XP/Vista users. If you go this route do read the DXVA Support page it contains important information on the best way to utilize dxva - so happens to be dependant on operating system.

I was able to run my Athlon 2500+ mobile /w AGP HD2600xt Radeon and it handled playback quite well for the Naruto subs anywhere between 5 to 30% using the haali codec (it's the only one that wouldn't stutter the dattebayo subs) on XP 32bit

Unfortunately for me the lunar anime dvd fansub of Macross Frontier was a different story though during the heavily subtitled music scenes with alot of talking took up to 80% cpu power or better with frame skips under the renderless vmr9 output method.

I bought into bluray over christmas and certain movies would drop frames so thats why I upgraded