Those damn acheivements

So my nephews and I are having a friendly competition to see who can get to 50,000 gamerscore first. Now sounds like a fun time but when I started this task I started playing a few games I normally wouldn't have played or wouldn't have played for long just for the acheveiments. Case in point, currently i'm playing Dark Sector and besides being on the last level i'm frustrated because I missed one upgrade. Now just wanting to finish the game I realized I need to go back and replay the game to get the case acheivement for a 15 more points to my total. Let's not forget to replay the game on Brutal for the extra 150 points. Normally I would have quit playing this game awhile ago because it's not that good but you know those damn acheivements.