Amazon is running an incredible deal - buy the Alien Anthology blu set for just 30 bucks while supplies last. Even if you don't have a Blu player yet this is a set you should buy for the future; the transfer of Alien is one of the best EVER, and the documentaries on this set are the industry-defining standard of what we should expect when classic films get released for home video. I mean, the docs themselves are worth the 30 bucks.
One word of advice: don't watch the movies before seeing Prometheus. I saw it yesterday, and while I cannot give you my full review I have been given permission to say basically that your expectations are already way too high. Don't raise them with a viewing of Alien, one of the best science fiction films ever made. Watch Event Horizon instead. But still buy tickets to Prometheus! It's gorgeous and the 3D is wonderful.