I believe that the program calibre automatically converts cbz/cbr files (which as you said are just images in a zip/rar) before transmitting them. So far I have attempted two comics on my 600. One was the manga Soul Eater, which is completely readable. The other was a graphic novel that had been scanned in and the text was nearly impossible to read. The manga certainly had an advantage because all text was typed in post scan (by the translator), whereas the graphic novel took a hit during the scanning process. I plan on running the graphic novel through some short of sharpening filter and giving it another shot this weekend.
All that being said, both of those items were natively printed in a format smaller than typical US comic books, which I have no experience viewing on my device. I would certainly try what schmolch said (keeping in mind that you can also use landscape mode which will show you half of the page at a time at the 800x600 resolution). You might even want to go one step further and convert the pages to gray scale if they are in color, since that can have a large impact on readability.