Here are some example topologies:
What you do with the application servers largely depends on what services you are running in the farm and how heavy they are utilized (i.e. it depends on your requirements). SharePoint 2010 will handle it's own internal load balancing with the service applications, so no need to set those up behind a load balancer. It just depends which application servers you have activated to run those services.
Some services such as the search query role, might actually be good candidates to run on your front end web servers. That way if your application servers are unavailable users will still be able to perform search queries.
Other examples of how you might split up roles on your application servers might be to have a dedicated search crawler on one application server and then other services on the other.
There are endless possibilities of how to use the application servers, it mainly comes down to how your farm will be used.
Regarding your SQL Server, even though you only have a single server in place, you might want to consider installing that machine as a single node SQL Server cluster so that if you decide to add additional redundancy to your SQL Server tier you only have to add a node to the cluster. Migrating from a non-cluster environment to a cluster environment could be a pain depending on how large your farm is.