Case-Insensitive Field/Index/Search

I need to have a (actually several) field that is case-sensitive: it will record text as is keyed in by users, respecting lower/upper case. But by the same time, I need to be able to search on this field in a non-case-sensitive way. It's going to be a fairly large database (1 million entries or more probably). Is there a simple and efficient way to do this by playing on field type, index and SQL search statement? I can think of creating a duplicate field that contains the same value but all in upper-case, but this will take up a lot of storage. Any better ideas? Thanks in advance. Yudong