MySQL performance tuning

Question

I would like to optimize my MySQL DB. I noticed some important points as follows:

1. Enabling Query cache.
2. Using memory storage. (I know pluses and minuses of this. Being I am having static tables, I need not worry about this facility's drawback. Before executing a query, I'll check if memory tables exist or not.)
3. Increasing key_buffer_size.

My question is, can I use both Query cache and memory storage? As well as what is the role of key_buffer_size?

Why am I asking? Because everything depends on system memory. How these 3 options will work together? Can I do these things together?