Kudos to you on your luck! I would say that you should take it back, simply because it's the "right" thing to do. However, when you think of these companies, you have to realize 1 thing:
1. They have more money than they know what to do with. Reason being? Mark-up's for profit. But, they aren't going to come out and say, "Oh, nice choice on the video card...we only paid half of what we're charging though!"
So with that in mind, I say to you....Keep it and enjoy it!