Weill - it's been a while now, so I don't remember exactly how it went down.
But I do remember trying everything to get a higher res and not being able to figure it out. It was limited to 1280x something.
I knew that my previous playthrough in XP had been at 1680x1050, so I checked online and sure enough, it seemed some others had the same issue. As instructed I went to the ME folder in Steam and ran the ME config program, and there it let me check 1680x1050.
The config box had not appeared when I installed ME in Win7; the game started automatically.
This isn't the only game where the config box doesn't come up. To run Call of Juarez in DX 10, for example, I have to insert the disk every time to get that option.
Live and learn.