Being a recording engineer (my "spare time" occupation) for a few years now, let me warn against using any "built in" microphone. Even a $30 Radio Shack Special will do a better job than most built in mics. Buy a condensor mic that doesn't require phantom power (phono plugs) and you should be able to use if with most devices.
If you have no other choice, do a few test takes to get a good signal without distorting. Pianos are loud, and have a tendency to overload some cheap systems. If you can adjust the input gain, set it to where the loud parts are about 60-70% or so of signal capacity -- this will allow some headroom for editing later.
If you can't do this, you'll have to put some considerable distance between the piano and the recorder to make sure there's no clipping.
As far as quality is concerned, I'd advise against recording with an mp3 recorder. The bitrate is simply too small to get a good recording out of it. Cassette tapes are ok if that's all you have available -- analog is generally better because there is no encoding (and sample loss) involved, though the additional noise can be a bit much.
My suggestion? Record onto a VCR. You may not realise this, but you won't get a better storage medium without spending a small fortune. Perhaps with a seperate mic going to the imput of your iRiver and the output going to the input of a VCR, you will get a very good result.
Then, you can transfer that to your computer via a 16 or 24 bit .wav recorder and make a CD of it -- no encoding loss that's noticeable. You'll have to dither down to 16 bit to make a CD if you record in 24 bit, though. Though you'd have a generation loss in transferring the analog recording to the computer, the end result will still be MUCH better than if you recorded straight to an mp3 recorder or cassette tape using a built-in mic.
Topic: Which one has better sound quality?