Split Fields into 2 Tables or All in One?

In setting up a member table in MySQL, I'm not sure if I should include all fields in the table or only the most common fields, leaving things like address fields to be placed in another table being that members aren't required to provide mailing address info and these fields may be mostly blank for most members. Will putting all member fields in one table be a bad idea over splitting the data into two tables: member account and member detail? It would be easier to have all fields in one table, but, if it means slowing down processing and more overhead, I'll separate fields into two tables. I don't expect to have huge numbers of member records, perhaps under 5,000 to start with. Thanks! Monty