First go to
http://audiko.net/ This site will let you easily create ringtones for your phone BUT what's better is that it stores all the tones everyone else has already made AND it stores them in multiple formats, one of which is the iphone. Chances are you can find any tone you want already made. FYI it requires no sign up, no log in, nothing.
When you go to
http://audiko.net/ you should see a "search" box. Type in the artist you want, in this case "taxi" and then hit the "find ringtones" button. You will be presented with a list that matches. Click one and you'll be taken to a page where you can listen to the ring first. It also shows you the number of seconds and how many KB.
Right under the player bar you use to listen to the song there are three options "download mp3 ringtone on desktop", "download ringtone for iPhone" and "For older models (amr-nb format)".
Obviously you'll want the second. Click it and the ringtone will be on your desktop already in m4r format. Click it to open it up and itunes will launch and automatically add it to the ringtones folder both on in the player and in the computer. You can then delete the file on your desktop as a copy now exists in the itunes/ringtones folder.
Next time you sync it will go to your ringtones on the phone.
Now if the tone you want ISN'T already there. Then on
http://audiko.net/ right above the search bar is an "upload" link. Click it and find the tone you want on your PC. Upload it and then then redownload it in iPhone format. Yes you can do this other ways that may be a little faster BUT doing it on audiko will let others quickly share in what you made. The more people using it the less work the rest of the community needs to do

.
(P.S. not trying to be insulting I'm sure you didn't need it broken down step by step but this might help anyone)