Thanks again EW - I was under the impression that connecting through standard sata cable would be a bottleneck and not take advantage of the 6 Gbps maximum transfer rate...so I'm still unsure as to what type of adapter will allow the fastest transfer rate
I haven't seen cable vs. M.2 slot tested for the same drive but if it's true it's not going to be a big effect.
6Gbps after encoding but before other overheads is 600MB/s and 2.5" SATA SSDs (cable connected) regularly hit 550MB/s data transfer (i.e. after all overheads). If there's any improvement to be had it's 5% at the very most, and more likely 1-2%.
One factor that would limit the maximum speed of your drive is that your motherboard's SATA controller only support SATA II, which is a ~270MB/s transfer rate, rather than SATA 6Gbps.
Whether that would be a bottleneck depends on the programs you're running. For typical home use the benefits mostly come from the increased random access speed and the reduced maximum speed is rarely going to affect things. Especially on an older computer like that where the CPU is more of a limiting factor.
If you did want to get the maximum out of it you'd need a SATA 6Gbps controller card.
You'll need one that can connect via a PCI-E x2 or x4 interface, also known as 2 lanes/4 lanes. The PCI-E x1 slot on your motherboard only transfers at 500MB/s. That means if you've got a graphics card in the PCI-E x16 slot it's a no-go.
You can go for a one with either an M.2 slot on board or one with SATA ports and an M.2 to SATA cable adapter.
Something like a Lycom DT-122 is what you're after:
Lycom DT-122 M.2 SATA SSD to PCIe HHHL Adaptor