Hi Neil,
I am using such an interface myself, a thing called Midisport Uno, and it works great. It really doesn't add any noticeable latency to your system, when I use it with the B4 (which supports ASIO and works with pratically no latency on my system) its perfect. With HW the delay comes from Windows XP stuff (ASIO can bypass the problems), but its still ok.
So, I can only recommend such an interface.
Greetings
Reiner