Hey, Liam Nolan, the Sony Readers (500 & 505 both) have 3 built in fonts: a serif (think Times New Roman), a sans serif (think Arial), and a monospace (think Courier). The way it handles fonts is to parse whatever font into the matching built in one. The only way to control what font you use beyond that is to embed the fonts in either a Reader sized (i.e. 6" diagonal that matches the Reader's display) PDF or an LRF (BBEB) file.
However, you should find that the Reader reads your RTF files just fine without conversion, but I don't know what font it'll choose for them.
I don't know about
batch TXT to RTF conversion, I'm afraid (though a google search will likely turn up a passel of them for you
), but you can convert them individually via word, wordpad, etc.