Selling my house

I have owned my house since 1989. My husband (who is deceased) and I bought it for $135,000. My realtor says I could sell it for somewhere upwards of $400,000. My fiance and his wife (also deceased) bought their house in 1973 for someting like $40,000, and our realtor thinks he could sell if for over $600,000.

 

We probably want to move to a different house. Should we sell one house before we get married, and then one after? If so, in what order? If we don't move to a different house, which one should we sell?