It could be saying 510 because the other 2MB is dedicated to something else. I mean sometimes A computer will report less because some of it was being used for the integrated graphics card, it went back up when I installed a AGP card. Anyway, installing RAM is pretty easy, HOWEVER make sure your computer supports more than the 512MB RAM, when I started installing this stuff I didn't know much about it, bought 2 X 512 MB RAM card and it didn't count it, found out later it didn't support more than 512MB. Also, if you only have two slots for your RAM you will need to make sure that they both aren't used. A lot of computers come with dual pairs of RAM so they both may be filled with 256MB, if your computer don't have more than 2 slots and they are both filled you will need to buy a bigger amount of ram to replace those chip already in there.
Since you want to play BF 2 It would probably be better for you to just replace them with 1 GB total new RAM Best thing to do is, check how much RAM your computer supports, check the speed and type of the RAM it supports (type meaning SDRAM, DDR SDRAM, etc..) and finally check to see if it supports dual channel, if so then it would be better to replace your RAM with two 512MB sticks of the same type (in fact the box usually will say 1GB 2 X 512MB). I hope that helps some, to find any of that information out you can call the manufacture of your computer