I'm in the same boat as Tom.
No matter which side of the DAC argument you are on (indiscernible by humans vs. noticable improvements,) the fact that people are arguing over it tells me that 'even if there is a difference' it must be a very, very minor difference or there would not be so much argument over it.
Therefore, I can safely say changing speakers will provide the biggest bang for your bucks without question. If money is limited, spend it on the sure thing first. You can always play with the DAC later on without the regrets of skipping the speaker change.
With great power comes Awesome irresponsibility.