You shouldn't try it. As far as I know downscaling hd content is really really intensive. Just get regular avi's or convert all the way down to wiz's resolution. No need to put 1900x1080 content on a 320x240 screen, eh?
(converting can be read about on a separate thread, there was one recently, I think what they ended up with was using handbrake to convert. I've watched only a single movie on the wiz, but that was a regular avi (640x480 I think?) and it worked with no problems. Of course you'll save on storage space if you convert.)