Personally I'd recommend getting yourself on the email list of estate agents. Tell them what you are looking for. Then I would also start viewing. That way you get a really good feel for what's around. We started viewing about a year before we were ready to out ours on the market because we wanted to stay in our village and wanted to know what we could expect to get for our money. Round here you then need to get your house on the market before you can make an offer because most things sell pretty quickly and vendors need to know you are a good catch, not sure if that's the same all over though. From reading this property section I've come to the conclusion we live in a bit of a property bubble here!
It depends on how boyant your local market is. If it is a quick selling area then by all means have a look at what you can afford, get some agents in for an estimate of what's yours is work, get solicitors quotes etc but if you are I a slow area then you must market first.
I would have a look at a few houses to get a feel for what is available. Then put yours on the market and start looking more seriously. However, few vendors will accept an offer from you until you have a buyer.