I have Earthlink DSL up here in MA, and it is fast... You might have a problem in the line somewhere. How far from the phone station are you??? THAT will determine how fast your DSL speed is. I am under 1000 feet from the station, and I constantly see at least the 1.5Mbps (peaking at close to 2Mbps) that I was promised (downstream with a 384Kbps upstream).
As for client software under OS X, you don't need any. Just configure the network preference pane to reflect what they have you use (such as PPPOE) and you are good to go.
I also have a Airport base station and network at home. The base station keeps me online 24/7 and the network allows me to use the printer from any computer (the printer sits on the network). I enjoy being wireless with the TiBook, since I can go anywhere within 150 feet of the base (all of the apartment as well as most of the yard where I live).
For those in the UK, wuJ is in the US, so it will do him NO good to hear what ISP you are using. 😛
BTW, W-_-W, those speeds you listed are pitiful. 😱 😛 😀