Is it allowed to talk with spouse during the sexual intercourse ? i have heard as its not good to talk. and is there any dua (invocation) to be recited before having intercourse every time and any specific time?
Regarding talking during sex, there is no clear ruling about it in islam that it is prohibited , haram or disliked. And you should know that in islam, Anything , that is explictly not considered haram/disliked, is halal or mubah in some cases as well.
Also, Allah says in Quran:
Your wives are as a tilth unto you; so approach your tilth when or how ye will; but do some good act for your souls beforehand; and fear God. And know that ye are to meet Him (in the Hereafter), and give (these) good tidings to those who believe.
So from islamic point of view, it is fine to talk ruling sex.
but from logical point of view, it is considered wrong to talk too much during sex, as it diverts your attention from pleasure to understanding the words. So you can talk , but better keep that talk specifically related to your enjoyment and your feelings during that pleasure.
One of the scholarly fatwa about same issue is here: http://islamqa.info/en/45597
And here is Dua to recite before intercourse:
بِسْمِ اللهِ اللَّهُمَّ جَنِّبْنَا الشَّيْطَانَ ، وَجَنِّبِ الشَّيْطَانَ مَا رَزَقْـتَنَا
In the Name of Allah. O Allah, keep the shaytaan away from us and keep the shaytaan away from what You have blessed us with.
Included a clear image of a dua,
And Allah knows the best.