number of hard drive bays in Alienware 17

As far as I know there are two for hdd/ssd. What I'm uncertain about is whether it comes with the actual enclosure and the necessary adapter. 

I can now confirm to you that the alienware 17 does come with the enclosure/adapter for the second drive bay. At least it did on the alienware I saw recently. It was ordered with a single drive/msata but did include the enclosure/adapter for a second hdd/ssd.

Yes, since I went through this process today. I added an ssd drive to the alienware 17 and made it the main/Windows drive, and am using the hdd that came with the laptop as the secondary/data drive. 

Basically I cloned the hdd drive onto the ssd drive. You can either do that by installing the hdd in the laptop (which is what I did) or by using an external enclosure. Once I cloned the drive, I swapped the drives (not sure if that was necessary): so the ssd is now where the original hdd was and I put the hdd in the secondary bay. Restated the computer and the first time it turned off by itself. So I booted it up a second time and it just worked, magically. BUT, it was still booting up from the hdd and not the ssd. So restarted the laptop, went into the bios and changed the boot order to make the ssd the first on the boot list; save changes and bam! Booting from the ssd and the secondary hdd drive also there in Windows explorer for me to use as my data/media/whatever drive. 

I ran a disc metrics application on both drives, and as you might expect, the ssd drive performed drastically better. The numbers were amazing on the ssd as compared to the hdd. You'll love it!

Good luck! 

Oh, your information is very helpful. Thank you very much!