For all practical purposes, the type of RAM you use will be determined by the motherboard you choose. This is because all but a very few motherboards are able to use only one type of RAM. I've seen a few that were able to use either SD-RAM or the more-expensive RAMBUS RAM back in the day, but they were the exceptions rather than the rule.
Most motherboards can handle more than one speed of RAM, however; and all motherboards can handle different amounts of RAM, up to the maximum that they support.
If your budget allows for it, I suggest you check your motherboard's documentation and use the fastest speed RAM that it supports.
For example, if you have a choice between DDR4-2133 and DDR4-3200, choose DDR4-3200 if you can afford it. RAM speed is one of the few things in a computer that almost always makes a noticeable difference. We'll talk about the amount of RAM a little further down this page.
If you're building a new computer on the same day I'm revising this page, chances are you will be using DDR4 RAM. That's the current standard and the fastest-available RAM on the market today.
When using dual-channel DDR, pairs of identical RAM modules work in tandem to greatly improve performance. Both the size and the speed of each RAM module in a dual-channel pair must be identical. I actually like to use identical RAM in all the slots: same size, same speed, same manufacturer, and same model. If I could get sequential sticks that came of the assembly line together, I'd do that, too.
How Much RAM do I Need?
Dollar for dollar, nothing will liven up your homebuilt computer more than endowing it with a respectable amount of RAM. In addition, because RAM is so central to your computer's functioning, always choose RAM made by a reputable manufacturer like Crucial, Corsair, Kingston, Samsung, or G.Skill. If you're a gamer or power user, also consider RAM coolers.
Every operating system has "minimum system requirements" for processor speed and RAM; but in my experience, the minimum requirements are grossly inadequate for the computer to be able to do anything useful. Instead, I suggest the following RAM amounts for desktop PC's running standard desktop software (word processing, spreadsheets, etc.):
Windows 8 32-bit: 4 GB
Windows 8 64-bit: 8 GB to 16 GB
Windows 10 32-bit: 4 GB
Windows 10 64-bit: 8 GB to 16 GB
Linux or other Unix-like system with X-Server, 32-bit: 4 GB
Linux or other Unix-like system with X-Server, 64-bit, 64-bit: 8 GB to 16 GB
If you use your computer for gaming, graphics manipulation, video editing, CAD/CAM applications, or complex mathematics, then I suggest at least doubling the higher numbers in the above recommendations for 64-bit operating systems. For example, the computer I'm using right now has 32 GB of RAM because I use it for frequent photo editing and light video editing, as well as lower-stress work like coding Web pages. My Linux laptop, which I use for light video editing and compiling binaries, has 16 GB. I'd actually use 32 GB if it supported it, but 16 GB is all the BIOS will support on that laptop.
My most recent homebuilt computer was designed and built specifically for heavy-video editing and transcoding. I designed it around the Intel Core i9-9900k processor and endowed it with 64 GB of DDR-4 RAM. That would be overkill for almost anyone; but rendering, encoding, or especially transcoding long videos can use huge amounts of memory. I chose G.Skill Ripjaws memory for that machine because in my recent experience, I've found it to be extremely stable and reliable under sustained load, and also to be a very good value. As of this revision (in April of 2020), G.Skill is my go-to RAM. It just seems to deliver the best bang for the buck for my needs.
If, for whatever reason, you're using a 32-bit operating system, then 4 GB is the most memory you can use. 32-bit operating systems can't address more than that. Even if the motherboard is capable of using more than 4GB, a 32-bit operating system won't see it.
Aside from that exception, it's really impossible to have "too much" RAM, up to what the motherboard and operating system will support. But there does come a point when you won't realize any benefit from having more RAM. If I were building a computer using Windows 10 64-bit to do word processing, check email, surf the Web, and watch streaming video, I'd recommend 16 GB and settle for 8 GB if that were all the client could afford. It's doubtful an average user would notice any improvement with more than 16 GB. But I'd also choose a motherboard that could support at least 16GB of RAM (and preferably more) just so it could easily be upgraded later if the user's needs were to change. If they wanted to do higher-end video editing, CAD/CAM, or other RAM-intensive work, I'd recommend 32GB to 64GB, or even more in a very few use cases.
Long story short, how much RAM you need is a function of how you plan to use the computer. If you're on a tight budget, I suggest that you skimp elsewhere (for example, by buying a less-fancy case), and also choose a motherboard that can support more RAM if you ever need to upgrade.
Mixing RAM Speeds
In one word: Don't. In theory, if you mix RAM speeds, all of the modules should clock to the lowest speed; but in practice, mixing speeds (and sometimes even brands) in the same PC can cause serious system instability. (And of course, dual-channel DDR must use matched pairs of identical modules.)
I also continue to recommend using identical RAM sticks in any given computer, including manufacturer and model, to maximize stability and minimize the possibility of conflicts. Although you theoretically can mix manufacturers, if you want, my experience has been that there are subtle differences between RAM that can affect stability. I've solved many problems with clients' computers over the decades by replacing all the RAM with identical sticks.