It depends on whether you will only listen to music.
I have an iPAQ 3955 that I have had for about 2 years. It serves well as a GUI Home Theater Remote ( aka Pronto ), GPS receiver, eBook reader, Dictionary, Translator, Media Player, and Time Waster.
I have the CF sleeve with extra battery and several 256, 512, & 1 GB SD cards. I use PocketMusic. It has some playability issues ( ends are clipped off some songs ) but it looks and feels like WinAmp.
If you get an iRiver or similar, you cannot expand the memory on it. I would get atleast a 1 GB model.
$400 will get you a used iPAQ 3955, CF sleeve, extra battery, and 1 GB SD card. I don't have a clue about the battery life since I have never listened to music for more than 10 hours in a stretch. With my
1600 mAh and 1840 mAh batteries, I am guessing a minimum of 40 hours play time since after 10 hours I only drained 30 % of the 1840 mAh and the internal was still 100 %. I have a spare 920 mAh just in case.
PDA will get you a much nicer player GUI but is pretty large compared to an iRiver. If you have a card reader, all you have to do is dump files onto the SD card from your PC