Ambition

When does ambition cross the line and become selfish ambition? The answer might be as simple as 'when you become selfish'.  But that seems like a much to black and white answer to something that seems, to me anyway, to be a very grey concept.

This has been bothering me for awhile now.  I have ambition, I want to do great things.  I don't want a mediocre or average life.  But does that make it selfish ambition because I want those things?  Is it bad to want those things?

We can get into the 'am I seeking to glorify myself or God' conversation here too.  And that to me is the crux of the whole situation.  I believe it is selfish, and wrong, to want to glorify myself instead of or above God; that's idolatry.  But is having ambition for myself the same as seeking glory for myself?  I'm not sure.

I'm greedy and selfish by nature and that is most likely why I'm struggling with reconciling my ambitions with a desire to glorify God and not myself. Any thoughts?