3-6C on various p3 systems I've tested it on... These systems put out roughly 25W of power.
On my system, (p3-650E@988, 1.8V) about 31W, i get a 4C jump going from Arctic Silver to Melcor Aluminum Grease. Haven't ever tried generic grease on my setup.
Also, Artic Silver application is very important. I found a new way to do it last weekend, and that is using your finger inside a ziploc bag to spread the stuff. This way, you can get very nice, thin, even layers. Depending on how I applied it, it was running 2C warmer at times.
Keep in mind that t-birds run almost twice as hot as p3s.
Mike