If you do go the i5 option, 6GB isnt an option, it actually makes it slower (due to channels or such), however for i7, yes go 6GB

Desktops definitely more cost effective, agreed. I'm from Australia so im not sure about parts over there, although i have seen a lot of forum builds and such with US parts, i reckon you could get an i7 860 or i7 930 (hyperthreaded for video encoding), or i5 750 (all of these are quad cores)... or a x6 1055t or x6 1090t, these are all the fastest for your budget for video editing, and all overkill for gaming. And with the left over money you could get a video card, say a 5850/5770/gtx 460/gtx 470/5870, ...please specify what resolution you wish to play your games at. Also in future i don't recommend posting these sort of threads on gamespot, but a computer site forum, tomshwardware, guru3d and such, they actually give out specific build advice, instead of vaguish advice like this :(. However this would go over $1000 with periphirals i reckon, so feel free to downgrade whatever you like.

Check out this thread:

http://hardforum.com/showthread.php?t=1512716