Naren. you can use her diary and letter(s) as evidence however they will have to be proved like any other peice of evidence and once proved they ought to effect the outcome of the suit, as far as Gtalk is concerned, things will get a bit complicated here, as to how will you present the contents of the chat before the Court, if you are planning to get a print out, it might be difficult to prove it during stages of sdducing evidence, im not going to go into proving of evidence too much as it is something which your lawyer will handle eventually, apart from the contents of such chat, if your wife denied that the id is hers, then again it will be quite difficult for you to prove that the id is actually hers and was made by herself and used by her goodself too when such alleged conversation took place! I hope you are getting the drift, right? Anyway coming onto the next part of your question:-
Your wife will get somewhere around one third of your disposable income, this income shall be determined by the court on the basis of evidence available on the record of the court. If the court cannot due to some reason ascertain your real income then it will assume a notional income and make such order for maintenance, this figure of one third of disposable income is something which had been decided time and again by various courts including Hon'ble SC, however i beg to differ from Galsober, you may have to pay maintenance irrespective of the fact that who caused cruelty, as far as my knowledge goes no such differenciation has been made by the legislature/ courts, if there is any citation saying otherwise do kindly mention it.