If you are looking to get the most gamer score you need to play a lot of games. New game cost anywhere from $50-$70
depending on where you live. The value of the games drop fast so by the time you are done getting all those achievements the game is worth half. Selling back is pointless,.

The best way is to use those video game mailing system like gamefly (US) GA (Canada) which for a very low price you can rent games for as long as you like, when you are done mail it back and get another game, these companies let you have more then 1 game at the same time too.. Perfect.. (for a more expensive membership)

So if you haven't heard about those check them out

The problem with these unless you live by the warehouse it takes up to five days for the game to arrive, but the wait is worth it.

What you do is get the 2 game out at a time membership its costs you around $20 a month, Rent a game
Play the game the full mission/storyline part of the games, once you have completed it and got all those achievement Go back on and rent another game. Now you have at least 5 days till your 2nd game arrives beat some side achievement till your 2nd rented game arrives and send them back the 1st game. Repeat over and over..

I could probably rent around 40 games a year depending on your commitment.