i am not sure about any diffrence in spped between low and normal, but i put all my crunchers on normal just the same. i tried seeting two of my rigs to high and it actully drop my speed down. So i wouldn't go up to high but i would say stay at normal. Maybe some one with some more experance can add a little more to this?
The performance increase is marginal if anything. At idle priority I've found SETI to use about 92-98% of the CPU time durring most activities (playing music, writing stuff, surfing the net, ect.) A bump to Normal kicks the percentage up to 95-99% with almost no change in other application responsivness. High will bring it to 97-99% (more often than not, 99% ) but you will be tearing your hair out with most apps not responding to you for a few seconds. Realtime... Well, I can't monitor the percentage because at that point, my computer "freezes". But it is actually crunching SETI (ran it for 30 mins on realtime to check), though plan on having to reboot just to move your mouse
In short, Normal is fine IMO (and where I like to run also) since you barely notice it and get about 2% CPU increase, though anything more is.... Well insane.
I've found no real difference in any priority settings. I do find that usability declines set to anything other than low. Even setting to normal is frustrating due to lag time switching between and using apps.