Getting a Pre-Approved letter is even better!  This means the Lender will do more than just ask you questions and give you an estimated loan amount.  Instead they will take you through the whole process by checking your income, debts, credit scores and such and when the process is done, the bank or mortgage co. will write you a letter agreeing to loan you up to a certain amount of money. 
It's better to do this process first because it puts you as a Buyer in a more favorable position as you present it to the Seller. In other words, if the seller receives 2 offers on their property (one being from you), they may see you as a weak buyer and chose the other offer if you do not present a pre-approval letter.