First off, figure out how much space you used of the hard drive of your last computer, and if you think you will need a lot more space. Point is, if you used 400 gig before, then having the two 500 gig drives might be worthwhile. But if you were only using around 250 gig, then two 500 gig drives are a waste.
That said, if its too much, try to return the extra drive if possible. But I wouldn't complain too much about the salesman, as it is your responsibility in the first place to say no to what you don't need. If you intend to keep both drives, I'd use the first drive for the OS and games, and use the second drive for data. That way if you ever have to reinstall the OS, you don't loose you data.