1 gig of ram would probably be enough, however i'd go for 2 gigs.
http://www.newegg.com/Product/Product.a ... 6820148163 for 20 bucks shipped, you can't really argue.
i doubt they will notice a difference with the 6800 ultra. the only difference they are likely to notice is the power consumption. the 6 series has no real benefit to helping video playback.
so go for 2, and i doubt they'll need the 6800