I should think you would get roughly 40-60 fps, depending on how busy the world is this may drop considerably. I've brought my very high-end computer to it's knees with minecraft before.
You can run a server at the same time. There are a few options on how to set it up.
Vanilla server:- Download the minecraft server exe file from
http://minecraft.net/download to an empty folder and run the file. This will generate all the files and folders it needs to run. You may want to edit a few of the settings within the server.properties file.
Modded server:-Go to the
http://bukkit.org/setting_up_a_server website and download the latest recommended build to an empty folder, this is not an exe file and will not automatically run if you double click it. Follow the instructions on the page on how to setup either a shortcut or batch file to start the server. When you run the server it will create all the files and folders it requires. At this point you still have a vanilla server, you may want to edit a few of the settings within the server.properties file. Also on the http://bukkit.org/ website are a large array of plugins/mods which allow you greater management control over your world as well as adding utilities to it.
The server is processor intensive. Something I'm not sure on is whether the Bukkit server is single or multithreaded, I think the vanilla minecraft server from
http://minecraft.net is multithreaded.