Not counting new games will need the power tha the 7900 series provides for max settings.
Doubt it.........when Doom3 came out, everyone said the same thing. 6600GT could play it maxed settings, maybe not with AA and AF cranked up completely, but it'd still give you pretty much maxed out graphics.
You over estimate how powerful these games will be. They wouldn't sell if the only way you could see the good eye candy would be if you dropped down a chunk of change on the latest and greatest card out there. Yeah you'll need the 79xx series if you want maxed out settings with 1280x1024 resolution 4x AA and like 8x AF and you just want it completely set to the highest the game allows, but the 7800 series will suffice for the settings that are maxed graphically...it'll only be the little side tweaks such as AA and resolution that'd you inevitably have to have lower, but I wouldn't think it'd cause a huge drastic change in visual appearance.