Have you got an account on http://www.GiantBomb.com ? If you do, you can attach every account you have if it's PS3 or Xbox or PC or whatever and it'll tell you how many achievements you have. If you made one quickly to see your total, we could pick how many achievements I need to win.
It's not tied to your Xbox gamertag though, but there's another PC game client for that. Games For Windows LIVE where it actually adds to your gamerscore.
Yep! On Steam. That's me there > http://steamcommunity.com/id/robertdonat If you click on the game stats bit to the right and then click on a game you can see the achievements and stuff I've got.