I use an AMD 5050e cpu and when I try a preview of a 720p@50Hz FFv1-video I have no chance to get anything usable, 576p@25 FFv1 is the limit, I think for my system. As I read here the video card is not important, so my onboard video (MSI K9N2 SLI Platinum, GeForce 8200) should be ok and it doesn't make sense to buy a graphis-adapter.

So which hardware could you recommend for 720p@50 and larger?

Check if you have the Xvideo acceleration enabled, type

xvinfo

in a terminal. If it says "no adaptors present", you need to setup the drivers for your graphics adapter