If I marry a girl and live with her my whole life I will get her as my wife in the next (eternal) life. Now if I marry a widow, what will happen? She will be my wife in the next life or her previous husbands' wife? If she remain his wife in her next life then what about me? And if I get her in our next life then what about that poor guy?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
If a woman marries more than one man in this world she will be with her last husband in Paradise. This is the preponderant opinion of the Muslim scholars. The evidence for this is the Hadith of the Prophet Muhammad (Salla Allahu Alaihi Wa Sallam) who said: "Any woman who marries again after the death of her first husband will be with her last husband". Imam Tabrani and Ibn Asakir narrated this Hadith and Sheikh al-Albani grades it as a sound Hadith.
Hudaifah said: 'Since a wife will be for her last husband so Allah made forbidden to the wives of the Prophet to marry after his death'. Some other scholars believe that she will be enabled to choose the husband from among those she likes the most. Then she will choose the best one who used to treat her honourably. Imam Ibn al-Qayyim was inclined to this opinion by the Hadith of Umm Salamah (Radhiyallahu Anha) in which she asked Prophet Muhammad (Salla Allahu Alaihi Wa Sallam): "O! Prophet of Allah, a woman marries twice in this world, for whom she will be in Hereafter life after their death? The Prophet said: "She will be for the one who had the best character. Then he said: "O! Umm Salamah (Radhiyallahu Anha) the good character takes both the goodness of this life and that of the Hereafter". [Reported by Daruqutni] Sheik Bin Baz said: 'This Hadith is not so sound.
There is a third weak opinion that she will be for the first husband. The Hadith on which this opinion is based is weak. Anyway, the first opinion is the most preponderant one.
As for the first husband Allah will marry him from among those women who died and did not marry or the women whose husbands will not enter Paradise. In addition, Allah will marry two Hurs (very fair females created by Allah as such not from offspring of Adam (Alahi as-Salaam)) to everyone who enters Paradise. No one will remain unmarried there.
Allah knows best.