To answer your question Shaun, as long as the immigrant enters with a legal visa of any kind ie; student, work, or tourist they can marry a citizen, file an I-130 and adjust status within the USA. Does not matter how long they have overstayed their original visa since an overstay is forgiven since they are marrying a US citizen. The only visa that can really throw a wrench in that route though is a fiancee visa because the beneficiary can only marry the person that petitioned them so if they decide to marry someone else they can not adjust status and receive a green card.