350 Gamerscore cap confirmed for XBLA, 1750 for Retail.
1. A retail game must launch with between 5 and 50 achievements for a total score of 1000 points. DLC addons of up to 30 achievements and 750 points are allowed.
2. An arcade game must launch with 12 achievements for 200 points. DLC addons of up to 9 achievements and 150 points are allowed."
This of course comes with a few exceptions, but for the most part this is the new standard Microsoft has put in place for developers.