I've bolded my comments next to each of the players you mentioned.
Do we have a Galacticos 2.0 on our hands here?
Hart is good, City (and England) would be better off if he stayed put and developed. England has a shortage (or lack thereof) of decent goalkeepers so if both parties wanted improvement I suggest he stay put. If you really wanted a good keeper, you can pick up Marcus Hanneman (a fellow yank!) from Reading on the cheap, and he's class.
Well Mahmadou is out for the season so he's ruled out and really we just need a half decent holding midfielder so Kompany can go to his preferred position at centre half back. Lassana is a decent choice and with Michael Johnson's injury problems for us and his lack of fitness at the moment Diarra isn't a bad choice until the summer.
I think you're slightly underating Aguero a bit. He's young and probably would take a bit of time to adjust to the Premier League but he's class and has great skill, a bit like a younger Robinho from what I've seen so I think he'd be a very good addition. A midfield with Aguero, Robinho, Ireland, SWP, and Diarra sounds good to me. I would even prefer him to Ribery but that's just my opinion.
Bridge doesn't really suck, he's one of England's best developing young talents and linked with a ton of clubs. Unfortunately as Fena said he's happy sitting behind Ashley Cole on the bench waiting for first team for Chelsea so he may not come anyway.
As for Hart I don't really think we're in the market for a keeper besides (realistically) Buffon, and that's just because he's the best in the world and could help Hart develop even more and potentially reach that level someday (By World Cup he'll be England's number 1 I'm almost sure of that). Great shot stopper and isn't a calamity like James, Robinson, etc. His only problem right now are crosses he isn't as commanding as he should be at that size but he's learning, nice string of games he's gone on recently as well in fantastic form.