Am I correct in assuming that if I just have each card mining on it's own (IE 10 x 400khash) there's a chance that one or more of those 10 may be trying the same combination of hashes and thus reducing my overall effectiveness?
No, that's not correct. The work you receive is always unique.
You can mine directly against bitcoind, or put a proxy in front of bitcoind (pool server) for better stats and management.
So what's up with things I've heard about people working on 'dead blocks' or blocks that have already been solved?
Sorry for the dumb questions, just trying to wrap my head around this.