Typically an apprasial isn't done by the seller when they choose to sell their home. However, it's not a bad idea. It could be helpful in providing you with any repairs that need to be done, and there would be no surprises at closing time. We paid $300, 15 years ago. How long have you owned your home? Check to see the appraisal cost when you purchased the house.
I've never sold a house, but I would start by calling a realator. They can tell you what they beleive would be a listing price for your home by comparing it to similar homes in your area Then you can deduct fees, what you owe on your current mortgage, and that will give you a good idea of what you'll have for a down payment.
If your planning on using the proceeds from your current house sale as a downpayment on a new home, then you would need to sell your house first. Also, before looking for a new home, you should get preapproved for a mortgage, that way you'll know what price range your new house should fall in.