This article focuses on the Speech technologies built into Mac OS X. Currently, these technologies are centered around the recognition of commands and the interaction between the computer and the user. By creating a real vocal interaction, they can be a lifesaver for visually impaired users.
In order to dictate text to a text editor you would currently need to use an application like Via Voice indeed. Although I haven't used it for a while, the last time I saw it, it seemed to be quite responsive and allowed the demonstrator I observed to talk at (almost) normal speed.
Considering Apple's commitment to help users with disabilities, I am sure that the Mac would be an ideal computing platform for you. Would you live near an Apple Store or Apple Authorized Reseller you can call and ask if they can perform a demo for you, so that you can see for yourself.
Here is a link to their "Accessibility (Vision)" website :
New cool accessibility features have already been announced for the next release of Mac OS X, "Tiger".
Let me know if this helps !