based on Dell knowledge article, only UEFI bios can boot from PCIe M.2 ssd. 730x bios is legacy only. Since 730x has four 3.5 hdd bay, one option is to boot from 2.5 sata ssd mounted on a 2.5 to 3.5 adapter.
  I am hoping to be able to boot Win 10 via a 500GB Samsung 960 pro M.2 SSD using a  Kryom2 PCIe 3.0 X Adapter M.2 NGFF PCIe SSD key with passive adapter. I realize that l most likely won’t be able to secure boot, but l thought l would at least ask. I still have the original HDD for the PC, which l plan on copying over to either a 1TB Samsung 860 SSD or 1TB Hynix S31 gold SSD, depending on the better deal. I don’t want to mess with the original drive because the all the original data on the Win 7 Home Premium 64 bit os HDD is in good shape.  I figure if I upgraded to windows 8.1, then Win 10, l would definitely mess up the partitions on the original HDD, and not be able to repair it.  I figure doing a clean install on M.2 SSD would make it faster for sure, and after l clone the OS, I’ll put the HDD in and maybe do some RAID storage . Eventually I will upgrade the motherboard and CPU, to modern, but things are tight right now and as a disabled vet, money is hard to come by. Still , my love of graphics, is forcing me to try to get back into gaming on the computer. I got a 4GB XFX GTS XXX Edition RS 590, real cheap(sub $200)from a  friend and supporter of veterans. I’ve been using Xbox for six years now, and while I enjoy using a controller, I know I can use it on PC also because l had one that worked with it back in 09…thanks for the help!
XPS 730x, SecureBoot using a Samsung 960 Pro M.2 via PCIe adapter