Catalyst plays back movies the way they were rendered. The High Quality setting in QuickTime Player is for playback.

Which codec you use really depends on what you are doing, how many layers you wish to playback and the hardware you have with your Mac. DV has the best performance - Macs decode DV very fast. However, DV can appear a little soft with text and other fine detail, as a lot of colour information gets chucked away. DV is also designed to work at standard definition. It has a fixed compression ratio (5:1), so the playback performance for DV clips is always the same.

I tend to prefer to use Photo-Jpeg for text or fine detail or for funny resolutions. Photo-Jpeg also has the advantage of having no fields - i.e. it's progressively scanned. Photo-Jpeg is a variable bit-rate codec. The compression ratio is determined by how busy the frames are and the quality setting. You can use Photo-Jpeg at 1024 x 768 or any other resolution. 50% quality works for many things. 75% can be a bit heavy going on your computer if you start playing several layers.