The amount of memory required depends on a variety of factors (e.g. document structure, embedded images and fonts, PHP garbage collection, etc.). Fabien has done quite a bit of work attempting to optimized dompdf without significantly altering the core architecture and can talk more about current requirements. In a future release we'll focus on further optimizations to the code with an eye towards faster rendering and less resources.
I've also been considering some options that could decrease the memory requirements significantly (though at the cost of speed).
However, we have a few major issues to address for the final 0.6.0 release and that is our focus at the moment.