You might want to look for a Trie implementation. That will certainly help with "words starting with XYZ" as well as exact matches. You may well want to have all of your data in multiple data structures, each one tuned for the particular task - e.g. one for anagrams, one for "by length" etc. Natural language dictionaries are relatively small compared with RAM these days, so if you really want speedy lookup, that's probably the way to go.
