These days, not much. Ideally, you would have enough cash for a 10% down payment, closing costs, and enough left over to cover two or three months of monthly housing expenses. That gives you a chunk of equity in your house upfront and makes the lender happy -- something that usually translates into a better deal. The trouble is, coming up with that much cash can be all but impossible for many first-time buyers. After all, we're talking $15,000 on a $150,000 loan or $25,000 on a $250,000 mortgage.
The good news is that lenders over the last couple of years have become increasingly willing to finance as much as 95% or even 97% of a home. While this sounds enticing, remember that small down payments have their price. You start with very little equity in your home as well as you will have a higher mortgage payment.