18743. Yes, it ends in a 3. I wanted something different.
Game with the most gamerscore? Halo 3, with something like 1550. All the map pack DLC being included with ODST boosted it a little.
Fully completed: Comic Jumper, Lego Pirates, Dragon Age: Origins (apart from DLC)
I don't (often) go out of my way to get achievements. I just play a lot of games.
And DLC achievements should be held separate to main game ones. Y'know, so you can still 100% a game without paying huge amounts more for it. That'll never happen - the desire to fully complete things is probably a huge selling point for most DLC.