At my office I have 3G and about two bars. I get between 0.5 and 1.2 Mbps down. Upload is around 60 kbps.
Here's a very simple page to check your download speed (no Java required):
http://speedtest.macbidouille.com/
There's an iPhone app to do this too. It's called Speedtest and it's free.
Tethering means that you use your phone as a "modem" for your notebook. The phone basically routs network traffic from your notebook to the cellular data network. Instead of browsing on your iPhone, you browse on your Mac over the iPhone's 3G connection. Unfortunately this feature isn't included with the iPhone. But jailbroken iPhones can be used to tether.