The difference between the three settings is not going to be very noticable in terms of work unit production. The main difference is how much it slows down the rest of your computer. I think the biggest improvement in SETI performance is when you change it from Low and Normal. If you change it to High, any increase is negligible, and you can't really do anything else on your computer.
On my Windows ME machine, when it's on Low, SETI uses about 90% of the CPU time. When it's on Normal, it uses about 96% of the CPU. On High, it uses about 98% and the computer becomes very unusable.
On my Windows 2000 machine, SETI is always using 99% of the CPU. 😀
So, I'd recommend leaving SETI Driver on Low, unless you're leaving your computer on overnight, when you might want to bump it up to Normal or even High.