It's not about Ghz count because new gen 1ghz cores are faster than 1,5ghz A9 cores. But say that they are cores of the same speed.
A Youtube video is a fair task for a core and I suppose it would make 1 busy. Facebook, app to block wifi and widgets, I'm not so sure because they don't sound too demanding and they're set to run in the background and shouldn't be able to request too much power.
A dual core would have an edge over a single core of the same speed because the youtube video would work in the foreground (1 core) and everything else in the background (2nd core), so even if those background tasks slowed down for a split-sec you wouldn't feel that while watching a youtube video and in worst case scenario your widgets would update slightly longer, but you wouldn't feel that. You probably wouldn't feel a difference between a dual core and a quad core here, though perhaps it could get the background tasks done slightly faster but that's only if those widgets took the whole core, for example being poorly optimized and downloading huge chunks of data at the same time and refreshing content.
A typical situation where I can see a quad core processor having an edge is when you:
- Download a lot of huge files via your browser (so many that it would take almost 100% of a core, because for some reason downloading files is CPU intensive on Android)
- Unpack a rar file
- Watch a youtube video (foreground task)
- Have a business program counting something.
At the same time. Most probably still it wouldn't lag even on a fast dual core because background tasks would be performed on the second core but you wouldn't get those downloads ready as fast and analysis would get done much slower, and it would take more time for the rar to unpack. Here a quad core would have a huge edge as far as the completion speed of those background tasks goes but still not much for your own youtube watching comfort. If you know that you do more than 2 really CPU intensive tasks like those at the same time really often and you want the background tasks done as fast as it's possible then a quad core is better for you.
BUT doing only 2 of those things you'd get them done faster having 2 faster cores vs 4 slower ones (for example Krait vs Tegra 3). If you were watching a youtube video and unraring a file it would unrar faster on a dual core Krait because the core assigned to that task is simply much faster. Even an occassional widget refresh would get handled in the meantime as it'd probably take only a little CPU time, and the outcome would still be better on a faster dual core.
Most of all if there was a single, demanding app running an intensive task to perform in the foreground it would get done faster on a faster single/dual core vs even a slightly slower (per core) quad core. Unless the app was really optimized for running on quad cores which doesn't happen often on Android.
And that's considering you have ICS. With previous Android versions dual cores had no real edge over single cores other than some more raw computing power.