Most things have been said but a few have been missed.
Regarding Ram your not only looking at the amount but also the speed. The speed of the Ram will determin how fast information can be written and retrived. Now adays you want to look for 1600 ram or above, but this will depend on what your processor and mother board can handle.
Ram is super cheap now, so really 16-32 is the norm now for video work. but again the amount used is determined by the motherboard and also by the operating system.
Processor wise eheath more or less has it. The faster the GHZ the better and also the more cores the better. How ever the higher the cache the better too. I5's are good, but if you look to upgrade go for an i7.
Somthing like a i7 3820 runs about $300, but its standard clock rate is 3.6GHZ which is pretty fast. It has 4 cores but has hypertheding. This means in programs which can use multi threading it acts as 8 cores significatly speeding things up. When hyperthreading can not be used it can overclock the used cores to 3.9GHZ. this means you can have a fast processor whatever situation the CPU is used in.
Top end machines will use Xeon proceesors. These are intels Server grade CPU's. They run slower then I7's but you can use them in a multiply CPU setup. This means youcan use 2-4 processors together. This means you could have a system with 32 cores. So the slower clock rate is made up for by being able to use more cores. They also can use ECC ram, this is error checking ram, which can be more stable hoever is more expensive.
Graphic card wise it used to be true that it did very little. Its still true for things like 3d, but its changing in video editing. Take CS6 for example. Premiere now uses the mercury playback engine. This means that the GPU can be used to take some strain off the CPU and create faster playback in the viewport.
However to use this there are some limitations, firstly it needs a Nvidia card. CS^has been updated so that certain ATI radeon cards found in Mac Book pros can be used, but unfortunatly you still cant use it on a ATI card in an Imac.
The card also needs at least 1GB of Vram. This does limit the amount of cards that can be used. Also unless you get a card which is offically suported by Adobe, you have to do a simply little change to a text file, in the premiere contents.
For after effects in CS6 there is now a raytrace render engine. This basically needs the same requirments as PP, so if your good to go in PP, AE will work too.
Hard drive wise it comes down to speed. Genrally now SSD are the faster and should be used for operating system, programs and files you are currently working on. This allows files to be read and written very fast when you are working on them. There is also the option to use a Raid system for this drive. This is where you use 2 standard hard drives and they work either mirrored or in conjuction. But its genrally accepted now that SSD is the way to go.
You would then want a larger hard drive for storage. a 7200 should be fast enough for this as now work will be done directly from this drive.
Hopefully that helps somewhat. Let me know if you have any other questions.
