I think bitcoin is the only coin you can actually use miners on. Most others are either not profitable or scrypt based. Unless you buy power hogging video cards.
You can only mine sha256 coins with sha256 asics. So far there aren't any asics for scrypt mining (for litecoins for example)
If you want to be able to change between different coins you should buy gpu's but gpu's suck for bitcoins because of the high difficulty.