Hardware and engine speed

I'm considering buying a new computer. I'd like to get a fast one, to get reliable analysis of chess positions quickly. Unfortunately, I'm a dummy of the highest calibre when it comes to computer hardware. If I come into a shop with electronics and ask "How fast is this computer?", what sort of terminology should I expect to encounter, and how can I find out how many kN/sec. can a particular computer calculate?

If you want good kN/sec you'll need a good SSD like a Samsung 840 Evo.  As for processors you want an i7 Haswell (4770k I believe) and Asus RoG boards like the Maximus VI Hero come with software for a RAM drive.  Allocate a few GB to it then save your Chessbase folder to it (RAMdisk is optional but games and programs load very fast). 

This is not true. Hard drive speed has virtually no affect on chess engines unless you are dealing with endgame tablebases.

Probably no one working at a retail computer store is going have a clue what's important when it comes to chess engines. A general rule is, you want a fast CPU, the more cores the better. For example, a CPU with "4 cores" basically has 4 CPUs on one chip. So a 4-core 3GHz CPU can, in theory, do 12GHz worth of work. Then you need a chess engine that supports multiple CPU cores (most of the top ones do now). More RAM helps, but 8 GB is probably fine. With both CPU and RAM, it only helps if you can get an exponential increase (i.e. doubling the amount of CPU/RAM). If your CPU is 10% faster, you're talking about tiny ELO increases, less than 5 ELO, so don't spend a ton more to get something that's only slightly faster. Along the same lines, unless you're going to increase the RAM from 8 GB to 32 GB or more, then 8 GB is fine. You can spend a ton of money on this if you want. You could get a computer with 30-cores that will do 84 GHz, but it will cost you close to $30k.

Frankly, I would just get something affordable. Even if you purchased the cheapest computer you could find in a retail store, you are still going to get highly reliable analysis. Spending twice as much can increase your "nodes per second", but it isn't going to increase the quality of the analysis significantly. For example, doubling the nodes per second (which will cost a good bit) will only add about 50 ELO to the ability of the chess engine. The engine is already rated at least 1000 ELO higher, so 50 ELO is pretty insignificant. Unless you're a super GM I wouldn't spend a ton on this. My iPhone 5s gets more NPS than my desktop computer.

I have experience with it.  It makes a big difference.  On the old build I'd get 750 kN/sec whereas now with seven processors I can reach over 9,000 in a middlegame and 20,000 in an endgame. 

I have experience with it too. I've developed chess engines as a hobby for 15 years and I've worked as an IT consultant for 10 years with computer equipment that costs more than your house.

An SSD is a hard drive. It has absolutely no affect on NPS. None.

You hit the nail on the head when you said "seven processors", that accounts for 100% of the speedup you are seeing. The processor and hard drive are different pieces of hardware inside the computer.

Having said this, if you want your computer to feel faster during general use, then an SSD is an excellent choice. The hard drive is the slowest thing in a computer, so getting an SSD makes your weakest link a lot less weak.

But like I said, a hard drive has virtually no affect when it comes to chess engines.