WiGhTkNiGhtt something to also remember is that as someone already pointed out years go by between E2 and E3. And then at least several months pass during the events of E3 - Padme says shes pregnant at the beginning of E3 and is not even showing any signs and by the end of the movie she's giving birth. That is a pretty good indication that we are not talking about days or weeks but months; months during which Anakin has persistent nightmares of Padme's death, where he is probably having conversations with Palpatine like the one in the theater, where he is becoming more and more disillusioned with the Jedi order and their restrictions on attachment, etc etc etc.
As Seductivpancakes points out the changes in Anakin's behavior is subtle. He is toeing, and sometimes crossing, the line between light and dark, but up until the confrontation between Windu and Palpatine, Anakin always finds his way back to the light. However, that confrontation gives Anakin the final shove to the dark side (IMO it was not only Palpatine's doing, Windu's solution to the problem also caused turmoil in Anakin).
Something else to think about, look how fast Luke goes to the dark side at the end of E6 - it turns out that it is the only way he can beat Vader. In addition, think about how fast Vader turns back to the light when the Emperor is killing Luke.
Lastly, all the movies (even E4-E6) sometimes do a poor job of indicating how much time has really passed. Do you really think that the hyperspace travel between Tattooine and Alderan in E4 took five minutes? There was even a thread on these forums not too long ago about E5 when Han and Leia decide to head to Bespin (Cloud City) and how long it took them to get there without Hyperdrive.
My point is that I am with Seductivpancakes on this one, it was not as big a jump to the dark side as you are making it out to be.