Re: What SSDs are available for this system? It appears that PCI Express would be faster than SATA, is that correct?  either 2.5" sata ssd or M.2 sata/nvme ssd.  for this older model pc it is probably more cost effective to get a 2.5" sata and install it like a second hdd via Dell 2.5 to 3.5 adapter. you are correct that NVMe ssd is faster than sata ssd (either M.2 or 2.5 form factor).  but you should also know that statement is assuming NVMe is using PCIe 3.0 lane.  Inspiron 660 has B75 chipset which only supports older slower PCIe2.0.  therefore an NVme ssd in 660 is backward compatible but running at slower speed than spec.
My Inspiron 660 desktop (2013) has been running slow. After running diagnostics (Dell, Intel, and Seagate), it appears that the issue may be the 1 TB Seagate HDD. I would like to add a 500 GB or 1 TB SSD and keep the HDD for less critical files. (I would backup the HDD to the SSD and then run Seagate SeaTools to fix the errors on the HDD.) What SSDs are available for this system? It appears that PCI Express would be faster than SATA, is that correct? I have seen M.2 SSDs on Amazon, but there is no M.2 slot on the motherboard, so I would need either an M.2 drive mounted in a PCI Express adapter or a dedicated PCIE card, does that sound right?  Thanks to anyone who can point me in the right direction. Processor Intel Core i3-3240 CPU 3.40 GHzInstalled RAM 8.00 GB two 3.5-inch drive bays for SATA hard drives, one Seagate 1 TB HDD installed Expansion BusPCI Express x1:Connectors: three Connector size: 36-pinPCI Express x16:Connectors: one Connector size: 164-pinPCI-E mini-card:Connectors: one Connector size: 52-pin OS: Windows 10  
Adding SSD to Inspiron 660