Fastest boot and load times are from SSD's, particularly one that uses PCIe x4. You need to be careful because some M.2 drives use SATA lanes instead of PCIe; so they are no faster than a regular 2.5inch SSD.

In my own build, I am using a 250GB Crucial M4 SSD and a 3TB WD Red HDD. I have the OS, applications, and a few games on the SSD. My main game libraries are on the HDD. Despite it being 5400RPM, it is still as fast if not faster than the 10 year old HDD I had been using before. I only bought a Red NAS drive because it was a great deal. I found it at a local store for $60 plus tax.

Anyways, depending on you patience, you can still game off a hard drive but I do recommend having an SSD to boot from and run your slowest loading applications.

EDIT: There are the WD Black hard drives that offer a little better performance in games but no where near an SSD.

@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. 

admiralWillyWilber what is your budget & required usage, also do you have components from a previous computer you want to re-use?

Thanks for help guys.

Someone gave this URL and have pretty nice reviews on it. Bought WD My Book

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx <- link removed.

Looking forward to receive it soon.