Here are screenshots of the RTF (Arial) and UTF-8 files on a Nokia 770 using FBReader. Note that you choose the font (and many details about how the text appears on the screen) inside FBReader. Both versions are "reflowed" to some extent, but it appears that the RTF version has hard line breaks. FBReader does not have line breaking (i.e. splitting words across a line break with a hyphen) for Polish, so I turned this option off. If one of the existing line breaking options does not work, you can request a Polish version from the FBReader developers - who are usually good about adding such things. However, this is less necessary in the 770's standard landscape mode than if you rotated the screen (which is an option in FBReader). I did not test them, but the other .rft and .txt files probably also work.
FBReader will also read non-DRM MobiPocket books, but the .prc file you linked to is encrypted.
FBReader is also available for Windows and Linux Desktop (see
fbreader.org), so you can try it out on a PC. The software and options are similar on all devices, but obviously the font choices depend on what is loaded onto the machine.