This issue can't really be seen on bencmarks for several reasons.
First,fps drops wont happen as soon as you start the game.You need to play a little before card runs out of video ram. Most benchmarks are not done that way.
Second,issue is only present with AA used,because AA uses a lot of video ram. If you dont use AA,then no,there is no game currently available which needs more then 512mb
And third,there is only handfull of games which needs more then 512mb. I am pretty sure these 3 games can't be run with 512 mb card and 1920 resolution with 8 AA,even 4x AA can cause problems :
Far Cry 2 , Fallout 3, Neverwinter nights 2
Fallout 3 is the worst. If you dont belive me,try running fallout 3 with max details and 8X AA with 512mb card and you wil be getting 5-10 fps randomly when entering vats system or just walking in the wasteland. Then drop down to 4x or disable AA and suddenly you get smooth 60 fps...so like I said 500 % difference.
And no,before you say its fallout 3 bug,I get same results in neverwinter nights 2 too. Far Cry 2...you need to play more...about an hour
Before you say it's my pc,I have tested this on my friend's pc's too.
Trust me,I know what I am talking about. Everything is fine if you dont run out of video ram. In the links you posted, Schmide, they didn't run out of video ram. That's why there is no big difference. They didn't use AA. And they didn't use games which are most video ram hungry ( I mentioned the 3 games I found to really cant work properly with 512 mb card )
Most peculiar thing is that Crysis seems to work just fine with 512mb. And it's for sure more hardware demanding then fallout 3,and especially then neverwinter nights 2. It just goes to show it doesn't even matter how demanding game is,just how well it's video ram usage is optimised