Unless you are paying all cash for your home, you MUST have a Pre-Approval letter from a Lender. Pre-Approval means you went through an entire application process. Pre-Qualification means you spoke with a lender, and based on what you told them (rather than showed them) about your income and assets, they estimated what you could afford.
The reason you must go the extra step is that Seller's and their Listing Agents will not take your offer seriously if they are not 100% sure that you can follow through on your offer. The Pre-approval letter lets them know that you presented tax records, bank statements, and other financial information, and a Lender committed to providing you with a loan (subject to the property and appraisal).
A Seller may actually accept a lower offer and skip over yours if they are not confident you can "perform". See my previous post "Getting Low Offers Accepted" for why this works.
Send me an email to Rob@SFisHOME.com and I'll send you the contact information for two lenders I work with on a regular basis who will do the Pre-Approval for free. Just imagine finding the perfect home, and NOT being able to buy it when and how you wanted. A pre-approval, and some smart negotiating will make sure you do get it.