For an English dictionary .txt file, you can use Custom Dictionary.

You can also generate a list aspell or wordlist with own settings.

Also you can take a look at http://wordlist.sourceforge.net/

Only english words: http://www.math.sjsu.edu/~foster/dictionary.txt

Most Linux computers have a file called /usr/share/dict/words with this list. Here's a copy: http://www.cs.duke.edu/~ola/ap/linuxwords

Thanks for the tip, vijayan121. It's better than the one I had for sure. It still has a bunch of "words" with numbers in them, proper names, acronyms, and words like "abd", which I'm pretty sure is not a word. However, like I said, it's better than what I have now and if need be, I can write a little program that strips out words with digits, capital letters, etc., and get a cleaner version. That'll get most of the dubious words out, but I am worried about the words like "abd". Before I do that, though, does anyone else have any leads on a cleaner word list? · Thanks. Definitely better off than I was before.

http://wordlist.sourceforge.net/ has a list of words. I would generally imagine that adding more words would not at all be computationally intensive (since its at worst the length of the wordlist amount of comparisons, and its probably faster if you are using a dictionary or some other optimized datastruture).