I don't know if the speed of the USB port vs. the parallel port would really matter a whole lot. The ECP parallel spec calls for transmission speed of 2MBytes/sec. If you were transferring 320x240 in YUV format at 30 frames per second your transmission rate would be about 4.6MBytes/second. So, with at least 2.5:1 compression you are within the ECP spec (as long as your cable between the DVC and the computer is 12 feet or less).

As far as the resolution of the device is concerned, I am pretty sure this is set in the hardware. I have been considering building my own MPEG encoding card and from the research I have done it appears that the MPEG-1 compression chips over the last few year pretty much came with 320x240 resolution. The later chips that do MPEG-2 and MPEG-1 seem to come with higher resolutions.