It should work. The VRAM is more responsible for the visual quality rather than speed. On normal 900p resolution medium should be fine too. High quality textures and great viewing distance in outdoor tank levels and air plane flying will be bad. Running around on the ground is probably better. In BF3.
MW3 is easy as it uses a lot of streaming but require comparably little VRAM. That should run on high settings.
Portal 2 I have no clue but I guess it will be somewhere in between those two.
Fluent gameplay won't be the problem of 512MB at todays games. It is more the actual visual quality you get even on stuff that should be the same settings. In the future and already today in many games you can simply not enable certain detail settings if they require the VRAM.
Higher resolution takes a bigger performance toll than on higher VRAM cards but 1440x900 should be okay. 1680x1050 depending on the game too.