For what you list, 8GB should be adequate. I ran with 4GB without any issue for over a year in my iMac i7 before I added RAM, primarily for running Win 7 via Bootcamp.
The tasks that need a lot of RAM are typically video editing/production, heavy-duty photo manipulation (not iPhoto or PS Elements), and professional page layout like Adobe Creative Suite. If you think that, down the road, you may want to delve into those fileds with this computer, by all means get the extra RAM.
I like to play flash games
I didn't try a lot of Flash games while I still had 4GB RAM but those I tried ran without issue and without causing a non-trivial number of page-outs
Things posted fast! See my edit made while we were all typing. Yes, Flash can hog but, functionally, my i7 handled Flash games and vids without problem when I had only 4GB RAM.
Thanks for your advice.