I've had my SB51G 3ghz machine for nearly a month now.
Performance has been extremely good (and stable - that's key). You can see my last benchmarks here
(top of the page).
The marks aren't as high as I anticipated, but this is straight out of the box without tweaks - it also has to do with my using pc2100 RAM.
As far as temps, right now, it's running idle with:
Sensor 1: 48C
Sensor 2: 54C
Sensor 3: 33C
On the overall - probably about the best system I've owned.