Bostrom says that machines can clearly have much better working memory than ours, which can remember a puny 4-5 chunks of information (p60). I'm not sure why this is so clear, except that it seems likely that everything can be much better for machine intelligences given the hardware advantages already mentioned, and given the much broader range of possible machine intelligences than biological ones.

To the extent that working memory is just like having a sheet of paper to one side where you can write things, we more or less already have that, though I agree it could be better integrated. To the extent that working memory involves something more complicated, like the remembered ideas being actively juggled in some fashion in our minds, I see no clear (extra) reason that machines would do a lot better. I personally don't have a good enough understanding of why our working memories are so small to begin with - clearly we have a lot more storage capacity in some sense, which is used for other memories.