i cant say i do a lot of editing (as im a bit rubbish lol ) but i have done a bit of video editing.
i had my comp built about 8ish months ago, and i have never managed to really make it burp.
Its using a 120gb sata 3 ssd and then a 1tb hard drive for storage, so programs run from the ssd like windows and such but all files/photo's/music get stored on the hard drive
Im using the inter i5 2.8ghz,(think its the 2500 sandybridge which comes clocked to 3.2) and then running 16gb ddr3 1600mhz RAM, never really used anything of the ram, at most it hits about 55%, so i would say 8-12 would be fine unless you going to go silly and expect it to render 2-3 hd pics/vids at the same time.
As for the processor once i seen it using 42% so really its more than enough for my basic stuff.
where are you planning on getting the comp, as you dont have to have all this from start, ram is really simple to add, but you will need to check the motherboard type that it can handle more, and also that their are spare slots available.
have a look here, will explain things more for you, prices are very good as well, if you fancy building your own.
http://www.overclockers.co.uk/