What is the primary function of the computer going to be? CAD? If so, see what THAT program requires to run efficiently. IF it works with hyper-threading, then go that way. You will essentially see 8 threads running in Task Manager with the i7 2600. I would opt for the i7 2600K (that can be overclocked. So can the i5 2500k but it does not have hyper-threading). If you plan on overclocking the CPU's , get yourself a CPU cooler (I would not stay with a stock cooler). I use the Cooler Master Hyper 212 Evo
. Price is right and it works. I overclocked my AMD Phenom II X6 1090t from 3.2 to 3.9Ghz. It was easy to install and there are videos on youtube to show you how.
SSD will speed up boot time and program access but at a cost of storage size. A lot of programs do not like being placed on anything other than a c: drive, so if you have a lot of them, consider the choice of a slower, but more space mechanical drive.
As for video cards, well, again, it depends what will be the primary purpose of the computer. I use ATI. Others, Nvidia. Choose your poison.
As for RAM, if the programs are 32bit, the most (IIRC) they will access is 2gigs of RAM. If they are 64bit, then what you have is what they will use. I have 16gigs because it was cheap to get. IF you run a lot of programs at one time, then the more RAM the better, with a 64bit OS. I will have VSP, PSP, web browser and a few other programs open at the same time and have no issues. I am using Corsair Vengeance low profile because if you get an aftermarket cooler, it might cover some of your RAM slots.
I am sure that others will chime in with some good suggestions.