.cbz is actually zip, not rar... hence cb*z*
As for the resolution, 320x240 is probably not high enough. Scan them in to look good on your PC monitor. You can always use Irfanview or something else to do a batch rescale later if you want something smaller (either lower res or higher compression) to fit on your Z.
My 860 is fast enough to rescale on the fly, so I tend to leave scans at full res. That allows me to zoom in if something's too small to see when I'm scaling to the screen. The CCS manga on my Z right now is about 600x800.
But the choice of image viewers for the Z sucks when it comes to comics. I'm subsisting on the standard image viewer, but a cbz viewer for my Z is first on my list of Z programming tasks