in short ADSL provides speeds upto 8MB, while ADSL2+ provides speeds of up to 24MB (which translate to about 16MB in the real world), it all depends on how close you are to your local exchange and what ISP you signed up to.
Some more details can be found here: http://en.wikipedia.org/wiki/ADSL and http://www.dslzoneuk.net/
Also choose one of the following routers and your money is well spend::