It won't be more then 4gb of ram. Filling and pushing around 8gb is going to take up even more processor time. My pc has 1.5gb dedicated video memory and 6gb of system ram. No game comes close to using even half of that. Crysis 2 devs just want it since they're used to the load everything in memory first then run the game pc mentality.
To go from 720p to 1080p is 2.25x more pixels. Double the texture resolution should be sufficient, 4x the texture memory. So that makes 1gb of video memory. I wouldn't be surprised if the next gen won't have more then 2gb of memory.
Hopefully they'll wait long enough that ssd drives become affordable and fast enough. The optical drive and hdd speed are the biggest limiting factors. Installing the game from a 6x blu-ray drive onto ssd drive should give fast random access to game data and not take an hour to install. Otherwise load times will be 4x as long as well. Loading a decompressing a level into 8gb of memory with current hardware would take, well at least as long as installing the whole dvd to your hdd on 360.
