I have fairly recently moved from a 256k/512k (up/down) connection to a 448k/8192k connection (real downlink speed about 6000k) and cannot see much difference - except on file downloads and only then from some servers. General web operation is little different.
I would move to the next speed up ("20mbits/sec") but only to get the ~1000k UPlink speed - useful if you are running a web server on ADSL
Against that, the old 256k/512k connection was uncapped, the 448k/8192k is capped at 20GB/month, and so is the faster one.