It's very difficult to answer this without spoilers. It's also difficult to answer about choice impact on ME3, as the game isn't out yet!
- There's a few choices that affect whether/which teammates survive.
- There's a major choice at the end which affects the Citadel's political power structure.
- There's a choice or two that affects potential allies/enemies available in future games.
None of the choices have an effect on teammates available in ME2. In fact, most of the ME1 choice outcomes feel like: "Remember when you made that choice? Here's how it's going now". Those outcomes probably will come to bear in ME3.
- Any or all of your teammates may die.
- There's a major choice at the end that affects Shepard.
- There's some choices that might affect the status of Krogan, Quarian, and Geth societies.
- There's some choices that might affect the Omega station's political power structure.
can the story across the first two games change drastically depending on your choices?
No, your choices do not change the first two games much. Mainly: if you make a choice that causes someone to die in ME1, then in ME2 you'll either not talk to them or some replacement character will be found for their dialog (with perhaps slightly different options).