an mp3 player has to have a transmitter in order to transmit the music to an external audio source. i don't know if the source you want to listen the music onto has to b mp3 campatible,though. Ur obviously an mp3 flas-fanatic like me and a lot of people, but u didn't mention the price range yur looking for. For flash-types, (256 or 512 as u stated) can range from about 89$ to maybe even 190$ or so. But i recommend the powerhouses of the flash- mp3 market:
The iRiver iFP 790- (of which i own)
Samsung Yepp T6X
Creative MuVo TX FM
Creative MuVo N200
Cowon iAudio U2
Most of these have very useful features like FM recording; tuner, voice recording and such. They should definetly satisfy u, as my iRiver did. And the price shud be good too.
Hope i've helped, respond to me of what one u bought