You are also supposed to use triple channel ram for the i7, unfortunately they cost significantly more, this kit will run signficantly faster but will cost you $48 more http://www.newegg.com/Product/Prod [...] inum%206gb
The problem I see with your build isnt that the 4850 is good enough or whatever its that your i7 processor will never get above an idle .
For the same money you could get
Phenom X3 720, or maybe and X4 945
2x2 gig of DDR3
4870 1 gig
500 gig hard drive [ seagate 7200.12 or samsung f3 are faster ]
sata dvd drive
a 22 inch monitor
its going to be better gaming experience , and will easily handle the other things you do
It wont be as fast encoding a video , and it wont be as fast running intense graphics rendering but you dont do that and so you shouldnt care about them
Outlander has a point. Unless you anticipate your processor needs increasing dramatically, you can build a more balanced system with an AMD 720BE, a stronger graphics card (e.g. 4870), AND upgrade to a 22" monitor at the same time.
Edit: Not to say a 4850 is weak. I tested my secondary rig recently, which is a 720BE. I play mostly older games (e.g. 2000-2005), and no current shooters, and they are all very playable on high settings at 1680x1050 (no AA needed at that resolution) using only a 4670. Background tasks included AVG (not scanning), and Teamspeak server and client, under Win7RC with 4GB of RAM.