People are talking about RAM, but not saying why.
When you run a program, your operating system copies the program code from your hard disk to your RAM. The same thing happens if you open a file, or view a web page: the thing you're looking at on your screen has to live in RAM.
When your RAM gets full (because you're running too many programs, or have too many documents open), your operating system starts taking stuff out of RAM and putting it back on disk (in a "swap file"). If those bits of memory are needed again, it will swap something else out, and then bring them back in.]
This is slow.
If your computer is swapping, you will benefit from more RAM. A symptom of swapping is: you alt-tab to a window you haven't used in a little while and it is slow coming up, and your hard disk is grinding.
So, RAM-wise, you want enough to stop your system swapping. Over this point, you may get benefit from more, but it will be much less. How much is that? For typical Windows XP usage, 2GB. I have read that typical Vista usage needs 4GB. Personally, I doubt you'll get any benefit from more than 4GB unless you're doing something really memory-intensive (and, also, you need to be running a 64-bit operating system to even see more than 4GB).
5Depends on the existing machine as well. A machine with 6GB RAM might not get much if any "bang" from an additional 6GB RAM. – Will Eddins – 2009-08-18T16:11:15.460
2@Guard True, the first link has a good post about RAM and OS sweet spots that should help clarify some of that information. – Troggy – 2009-08-18T16:20:19.547
Ram doesn't increase performance directly, it just boosts the number of programs that can run at once, and makes them smoother (especially ones with large data storage requirements like picture or video editing). – RCIX – 2009-08-24T01:17:59.840
I'd rather have 2gb of DDR3-1600 than 4gb of DDR-400. Storage space can not be equated with speed. – Breakthrough – 2009-09-12T01:29:15.333
@Breakthrough True, but you will never be upgrading a motherboard that supported DDR 400 and DDR3 1600. We are buying a new machine here, so good chance he will be buying ddr3 anyways. – Troggy – 2009-09-12T02:09:10.120
I do like what others are saying about ram speed. With whatever choice you go with, also make sure you get the highest supported speed of RAM with the machine you buy. – Troggy – 2009-09-12T02:23:52.687