The difference between compressed files above 128kbps and lossless isn't so much in overall sound quality,but rather artifacts that you have to listen to carefully to hear. 256kbps is at a level where these artifacts aren't going to be audible for most people.
If you want to test between them, here's a thread you might be interested in: http://www.head-fi.org/t/653438/high-bitrate-lossy-vs-lossless-test
The top link in my signature has comparisons between 320,128, and lossless as well but the samples probably aren't going to be as complex as bigshot has, and I don't include 256.
As far as youtube goes, I believe some old youtube vidoes have crappy audio encoding. But newer ones can be decent.
Here's a chart of youtube bitrates:
|Resolution||Audio Bit Rate||Compression|