I agree with pparks
Keep in mind that the HDD/SSD for multitasking is a very small part of what get's used in the PC over all.
It is practically insignificant depending on the processes of the software running.
Multitasking relies on RAM and CPU and Bus Speeds the majority of the time.
Once things are opened, the HDD/SSD have very little to do at all with the software etc.
Unless it's something like a database, or heavily used file server (like parks stated)
There is not much that the normal consumer runs daily that needs a lot of heavy disk access.
There, over heavy use of multiple users, you want the access speed of high end drives.
SSD at this time is not the choice of course. But, HDD's and Raid setups is.
SSD as they get bigger and cheaper will replace HDD's in File Servers. Terminal Servers, Database Servers, etc.
Raid will only be used for redundant drive failures, etc. not so much for speed at that point.
Especially once VDI get's utilized more. VDI is the wave that is coming.
In a home environment though, parks is exactly right.
I know I went a little off topic, but it's relevant.
As for RAM,
Performance is what that will give you in a PC that has little RAM to begin with
Keep in mind, that a lot of people don't suffer from this any longer. It is an outdated idea now.
Anyone with a modern PC today, probably isn't running with less than 4G, generally. Not everyone, but my guess is a lot. And they should be. No modern PC bought today should run with less than 4G of RAM, imo.
So adding 2G+ isn't going to give anyone a real performance boost.
Older PC's (4+ years) with only 2G of ram, will see a performance boost by adding 2G. Not huge, but enough to notice.
In gaming PC's, where I see a lot of people make mistakes is, dropping 16G of RAM and buying a mid level GPU.
That is a waste, I will always recommend, 6G RAM total, put the rest of that money on a better GPU, that is the bang for dollar.
If you are doing Audio/Video, Virtual Machines, Programing Large Databases, etc. You want RAM, lots of it.
8G would be the minimum I would recommend on the low end and that would only be if you were going to add more sooner rather than later, 16G Recommended as the primary choice for starters. Going up from there would depend on what you were actually doing and how much.
Rendering a 60 Piece Orchestra will go a lot smoother and faster.
However, this will also depend on the software you are using, and how well it will utilize the system over all.
Virtual Machines, will suck up massive amounts RAM to run stable and allow for multiple users to access things
And again, this depends on what you are doing with it. So, 4G to 8G may suffice for the hobbyist, recommend 6G to 8G though.
In a home environment, you are probably just playing around with things,
but you will get better performance out of the 3 server and 10 clients you may want to play around with.
Keep in mind that what I stated here are all my personal opinion on general guidelines and it can change depending on what specifically each individual is going to be doing over all and how much of it. Some will be quite satisfied with less, but depending on situation, I would never recommend less. Less is not more in the tech world, that is only valid in culinary arts.
In the world of tech, very little is written in stone though.