TCPMP is a amazing (best) PDA player for both audio and full screen video (AVI as well)
You can reassign any of the button for the PDA
You can customize the equilizer & video quality (brightness,contrast,...)
I set my PDA to power-saving most of the time, and it works smoothly (at 208MHz) on playing mp3 & it stutter a little (at 208MHz) on videos.
If my x50v is in auto-mode, it sometimes go down to 104MHz
I assign a button to open TCPMP (Settings->Buttons)
& use the same button to turn-off/toggle the screen (TCPMP->Option->Setting->HotKeys(menu)->ScreenToggle)
It uses minimal amount of the battery by turning-off the screen.
http://tcpmp.corecodec.org/download