I still think that 512 is too big for the OS.
My pesimist guess is 1GB of RAM (splitted as desired or unified) with 256MB reserved for OS, leaving 768MB free for the games.
My optimist guees is 1.5GB of RAM (splitted as desired or unified) with 512MB reserver for OS, leaving 1GB free for games
Both options are an increase over current systems and can be achieved at a reasonable price.