> Most of your 101 tabs should be idle, not active, so my guess would be "not much".

Keep open in the background a few js-intensive websites (e.g. gmail, twitter) and your tabs will be flickering like the proverbial Christmas tree. Depending on your hardware, the paradigm difference can be very noticeable (also, have fun on a laptop's battery).

But Chrome - and any sensible browser, really - will scale down JS execution on pages not in the foreground; you can see this in for example Piecon (http://lipka.github.com/piecon/), which will only update the favicon in your background tab bar at half the speed (if that).