You can make the Nexus S somewhat usable but you have to carefully manage it. Anything that syncs in the background slows it down and you get awful lag. So no Chrome, no Facebook app (just use it through the browser), and disable Twitter sync (just refresh it manually when you open the app).
I think what slows this phone down so much is 512 mb of RAM, and single-core processor. So I would look for something dual core and 768mb-1gb RAM. What country/what carrier?