I say sell both sticks and then buy 2 matching sticks of 1GB modules for a total of 2GB
Or he could keep the sticks and buy two sticks of 512MB for 2GB in dual channel. Or he could keep the sticks and buy two 1GB sticks for a total of 3GB in dual channel. I mean seriously how much do you think he is going to get for two used sticks of 512MB ram? Not much.