Does anyone know the best way to convert wma and ogg files to something
iTunes can handle?

There is:

http://www.easywma.com/

and its ilk, but I'm not sure how it goes about it.

In the past I've converted the files 'up' to wav, and then 'down' to the
native apple lossy format, selecting a slightly higher bitrate than the
original. Might this be the best way of going about it, or doesn't it
make much difference?

Thanks, Rob