Of course, as only one device can access main memory at any one time, having a faster bus risks causing memory related delays, that a wider bus can avoid.
I don't understand this at all. If you need to transfer 1GB of data, what difference does it make if you do so using a 256it @ 500MHz or 128bit at 1GHz? They have the same amount of through put, so what difference does it make?
It's actually a doubling, not quadrupling. Other than that, everything you said is right. A 256 bit bus of GDDR5 will be equal to a 512 bit GDDR3 bus, roughly speaking (eg HD4870 vs GTX 280).
When you look at video cards, why do they say 900MHz, 3600Gbps? Sure seems like its 4 times faster. (wait, I bet I know the answer, actual vs effective speed, or you have to take DDR into account.)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102801