Alienware 17 Base M.2?

The Alienware 17 has had several releases (17 R2, R3, etc.), and without that information I can't tell you what sort of connectivity options you have.  But if it turns out that 2.5" is your only option, I'm personally a big fan of the Samsung 850 Evo. It's fast enough to max out a SATA 6 Gb/s interface.

According to the Specs PDF available for that system on support.dell.com, it has an mSATA slot, two 2.5" drive bays, and the option to use the optical drive bay as a third 2.5" bay, though you'd probably need a caddy specifically for that purpose -- so you've got quite a bit of flexibility there.  Unfortunately it's not new enough to have an M.2 slot to use the latest and greatest NVMe SSDs, but modern SATA-based SSDs can still be quite fast, and the extra flexibility of an mSATA slot is nice because it would allow you to install an SSD there without taking up a bay that could instead be used for bulk spinning storage. The Samsung 850 Evo I mentioned earlier is available in both 2.5" and mSATA form factors, for what it's worth, but obviously I would recommend opening your system to see what you're currently using and what's available before buying anything.  That's especially true for the 2.5" drive bays since Dell doesn't always include the brackets and cables necessary to connect 2.5" drives in those bays if the system wasn't ordered with drives IN those bays to begin with. In that case, you'd have to order them from Dell Spare Parts, or you could just take that as another reason to use an mSATA card instead.

In addition to the above, I just realized I forgot to answer one of your original questions.  Yes, upgrading to an SSD would absolutely, positively, 100% be worth the cash.  It is by far the best performance per dollar upgrade you can buy, except maybe memory ONLY IF you're actually running right up against the limit there.  Otherwise, a 5-year-old system with an SSD upgrade will be MUCH faster for everyday tasks than a brand new system with a spinning hard drive.  If you've never used an SSD-equipped system, it is a completely transformative experience.  Just as one data point, I got my first SSD back in 2010, and it dropped the time from shutdown to the Windows login screen from 40 seconds to 13 seconds -- and that was the speed upgrade available 7 years ago.  SSDs have of course gotten much faster since then, while spinning hard drives haven't improved much at all in that time.  With modern SSDs, it's not uncommon for Windows to load in about 5 seconds, and logging into your account and loading a typical set of startup applications is usually maybe another 5.  From there, applications just sort of appear when they're launched rather than really loading.