<p>I am using 3D maths in my application extensively. How much speed-up can I achieve by converting my vector/matrix library to SSE, AltiVec or a similar SIMD code?</p>

Most likely you will see only very small speedup, if any, and the process will be more complicated than expected. For more details see
The Ubiquitous SSE vector class
article by Fabian Giesen.
The Ubiquitous SSE vector class: Debunking a common myth
Not that important
First and foremost, your vector class is probably not as important for the performance of your program as you think (and if it is, it's more likely because you're doing something wrong than because the computations are inefficient). Don't get me wrong, it's probably going to be one of the most frequently used classes in your whole program, at least when doing 3D graphics. But just because vector operations will be common doesn't automatically mean that they'll dominate the execution time of your program.
Not so hot
Not easy
Not now
Not ever