If you are getting 50-52kbps with your Lucent modem why change?
That's just about the absolute limit for a 56k modem and most people would be over the moon with such a performance.
I have a Diamond SupraExpress 56i V Pro and 52kbps is the best connection speed I've ever enjoyed and I live just half-a-mile from the exchange. I get 49.2kbps with Tiscali and 50.7-52kbps with Pipex.
A pal who lives further away has a ?9 basic modem and also gets 52kbps...:-)