Well pouring the oil in the engine will contaminate it as well, but yes of course you have to change the filter.

Drain and fill with however much it takes to get to the line on the dipstick. I buy the gallon or whatever size at Wal-Mart, 10W-30. 2 cars, I try to change them around the same time.

I buy my filter at NAPA, their store brand is a Wix filter, which is supposed to be pretty good. I found a few oil filter review sites online that list Wix as one of the better quality filters for the money.