This is sorta a continuation of when I said that replacing the boot HDD in PCs with an SSD usually solves the problem of an unbearably slow PC. Also, I should probably start a blog or a new thread, but a) I can't be assed to, and b) I don't see myself updating it often or regularly enough for it to really work.
The thing with getting a new SSD as a boot drive is that you can guarantee that a system booting off of it will at least have acceptable performance, even if you buy the cheapest one. HDDs promise no such thing. Were you to go for the cheapest HDD available, you'd almost certainly end up with a system that would be bottlenecked so badly, that even pairing it with an
overclocked i9 running at 7.7 GHz being cooled with liquid nitrogen would make a sloth think it's slow. And as for finding a HDD the speed of an SSD... good luck. They stopped making high speed (high speed as in the platter itself span at, like, 10000 RPM instead of 5400 RPM) HDDs a long time ago once people caught on to the fact that SSDs are just better in this role.
Personally, I have seen an Alienware laptop with an i7 and a GTX 980M being brought to its fucking knees by a slow hard drive. It was like it was begging for death every time I tried to run anything. When I had it replaced with my old SSD, it suddenly turned into the fastest laptop in the workplace it was in.
Just get an SSD for a boot drive. Any SSD is better than a HDD when you're putting your OS on it. If upgrading an old system, just make sure that the PC you're planning to put an SSD into actually has the option to replace the existing storage device; I've seen (on Youtube) laptops that have their storage soldered straight to the motherboard.