@Adambarton can you give us the make/model of your motherboard?
Also do you have any PCI Express slots available for use on your motherboard?
With your processor being a 1st gen Core i7 I worry that your SATA ports may be SATA2 instead of SATA3 and possibly not support AHCI which many solid state drives require for their garbage collection "Prevents slowdown over time"
I agree with those above, a PCIe 4x M.2 NVMe Solid State drive would be the absolute best, but may not be supported in your system at least as a boot device.
Usually at this point I'd suggest a Samsung 960 Pro & a PCIe Adapter card, but I don't want to suggest something you possibly can't use.
For transparency I use the following.
Samsung 850 Pro 512GB SSD for disk intensive games
Seagate Momentus XT Hybrid Gen3 drive for less disk intensive games.