A general rule of thumb is that it’s never a bad idea to stuff in as much RAM as your motherboard can support. After all, one can never have too much memory in a PC (or a laptop for that matter).
Do I believe in this rule of thumb? Yes, but with this condition: Stuff as much RAM in your computer box that your motherboard will support, as long as it’s the correct type.
Now I know, these days you basically can’t get RAM wrong as far as physical fit is concerned (especially since they’re all keyed now), however there are still a few considerations depending on the memory’s speed, power consumption and the operating system itself.
OS and architecture
A 32-bit architecture can address a maximum of 4GB RAM, however on a PC, the only OSes that can address the full 4GB are UNIX and Linux. Windows XP cannot due to pagefile "goodness", as it only allows a maximum addressable 3.2GB. As for Vista and Win7, the same limitation exists.
64-bit architectures on the other hand can handle as much RAM as the motherboard will physically support, be it UNIX, Linux or Windows. This means if you have a motherboard that can fit in 24GB RAM, the OS (as long as it’s a 64-bit installation and you have a 64-bit CPU) can address the full 24GB.
If you’re wondering "How high can I go?" with consumer motherboards concerning how much RAM they can physically hold, currently the answer is 128GB. Yes, ridiculous, but available. The RAM is installed via 8 240-pin RAM sticks at 16GB a piece to get that 128GB total.
Is 128GB within what Windows 7 64-bit will support? Yes. The Win7 64-bit OS will support up to 192GB RAM – as long as it’s Professional, Enterprise or Ultimate Edition. If you have Home Premium, that supports up to 16GB. If Basic, that supports up to 8GB.
Should you go with the fastest RAM available?
Generally speaking, no you shouldn’t.
For example, this monster motherboard that can have 128GB of RAM stuffed into it supports DDR3 speeds of 1066, 1033, 1600, 1866, 2133 and 2400. Anything above 1600 is overclock-able.
If you’re building a gaming PC, then it’s par for the course you’re going to have a better-than-average cooling system installed. In that case, yes it’s OK to go with the max-speed 2400 and overclock it because you have the cooling means to keep everything under the hood from turning into a volcano.
If you’re not doing the gaming PC thing, safe territory is to go with 1600 instead. Let’s say you were building something for use as a powerful HD video editing suite. It’s highly unlikely you would get any real advantage out of overclocked RAM compared to non-overclocked. What matters more is just the physical amount of RAM you have available for rendering super-high-quality Blu-ray spec (or greater) video files. The 1600 will be plenty quick and suit just fine for this task, and the cooling system used could simply be 4 to 8 high-grade fans (don’t forget the CPU and hard drive coolers) instead of other means like gamers use.
Another reason to go with 1600 is availability. A single 16GB 1600 stick is $100 (slightly cheaper elsewhere but $100/stick is generally the current going rate), meaning $800 will get the 128GB to stuff up the aforementioned motherboard. As for what the 2400 costs, I couldn’t even tell you because I couldn’t even find one for sale. If you can find one, then please, post a link to it in the comments because I’d like to know, too.
At the end of it all, for most of you out there, if you’re running a 64-bit CPU and have a 64-bit OS running, the high-speed non-overclock-able RAM is the best way to go.
Final notes for those thinking of buying a new motherboard soon
Getting a ‘board that can support 128GB is, obviously, quite ridiculous – even for a gamer. Furthermore there’s not too much choice when going with ‘boards that can support that much RAM.
As for how much RAM you should stuff in there, going with the full 32GB is a waste of cash for most people. A reasonable amount to start with – especially if budget is a concern – is 6GB. The 64-bit flavors of Win7, UNIX or Linux run quite happily on that and is very speedy. If budget isn’t a concern or you simply have the cash ready to spend, go with 16GB. The 32GB limit is normally reserved for hardcore gamers and those that need super-fast video rendering suites. For everyone else, 32 is just overkill. You could even say 16 is overkill.
Personally, I run 8GB in my PC. The ‘board can support up to 16GB, but I’ve never even come close to utilizing all my RAM no matter how much I throw at it (and believe me, I’ve tried).
"If that’s the case, shouldn’t I just go with a 16GB-max ‘board instead?"
No, because there’s less choice there. Even if you never use the full 32GB, you’ll get more options with the 32GB ‘boards currently.