Chain really quite stretched, should I change cassette at the same time?
The easiest way to tell is simply to install the new chain on the cassette. If the cassette is worn from the old chain, it will skip when you apply a decent amount of pressure (standing up will usually do this in your big ring and half way down your cassette). If it does jump, you'll need a new cassette. It's really as simple as that.

I would be surprised if you need a cassette. The lower model cassettes are made of steel and don't wear easily. (My current cassette, a Shimano 6700 is now 12,000 miles. I change chains about as often as you did and I do keep it well lubed).

You may notice a bit of noise when you change chains. I notice this also and it usually goes away after some mileage. If not, you can always try adjusting your rear derailleur a bit to see if that helps.