I'm encoding this on a relatively fast computer to be played on the PoS imacs at my school. I just made a .mov h.264 ~1k kbps, but I'm planning on trying more then 1 type. Also, I have too much unclosable crap to accurately gauge how much cpu% of mine each file uses.
I know you said no install but I am afraid that can not really be done, not much experience on a mac but I suggest ffmpegx: http://www.ffmpegx.com/ it is a port of ffmpeg (pretty much the de facto freeware encoder) and will be about the best you can do with it.
As for what to encode 500MHz is a bit slow for most things xvid at anything up to DVD resolution should work but remember not to use postprocessing when playing back and add it in once it is working.