1)If your tape recorder has an external microphone input, and you have a electric instrument, you can use the recorder's microphone input as a "line in" input by connecting your keyboard directly to your tape recorder via the external microphone jack.
You will probably have to get some phono cord converters and you will need a line splitter or else you will not be able to hear what you are playing.
The advantage of doing this is the recording will be free from any external noise but the piano, the disadvantage is you can't use it for singing as you play and also that you have to be very careful about distortion; what the tape recorder picks up may not be the same as what you hear in terms of distortion level. Experiment with sound tests and see what works before doing serious recording. What you hear through the monitor is not necessarily what you will get on the tape, so play a note or chord and check your sound and distortion level before recording.
2)I've found when I upload music from my tape recorder to the computer and convert them into Ogg Vobis, I lose my sound level. I decided to try to overcome this by using a guitar amplifier. I connect the amplifier to the tape recorder's headphone socket using a phono plug and a converter, and the concept is that I can get a better level this way on the Ogg Vobis files. I havn't actually tried it yet, but I'm hoping for good results when I do.