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Re: I have an Alienware 17 R4. Does it have Pcie interface to use the newest generation pcie ssd's???

No worries!  Bear in mind that M.2 is just the physical form factor; M.2 SSDs come in both SATA and NMVe flavors.  If/when you do look to upgrade, your 17 R4 will support the faster NVMe drives!  The performance difference here is just much smaller than between a SATA spinning disk and SATA SSD - but still noticeable.

Your Sandisk X400 is SATA, not NVMe.  I would not suggest running out and replacing it just for the performance boost though.  If/when you feel like you need more storage space, it would be worthwhile to consider replacing it outright with a large NMVe stick instead of just adding an additional drive - you'll get the most benefit when having the operating system live and work on the fastest storage possible, much more so than when using one as just a storage or games drive.