If they're using Mac OS X, you'll have less file name issues. OS X has a more similiar set of disallowed characters (due to the unix file system) to win than OS 9. Also, OS X is generally a lot better about file extensions - they're required in some places.
If you go in with a pretty open mind (ie, don't call them "mad mac'ers") you'll probably find making your PC work with a Mac (again, OSX here) is similar to making it work with a Linux machine.
And yes, a jpeg is a jpeg regardless of platform, but one 'gotcha' if you're doing technical work: OS X (Like all *nix platforms) encodes line endings diferently than windows does. Its something like: OS 9 uses CR, Unix uses LF and windows uses CRLF.
If you've got more questions after the job starts and your co-workers are as clueless about PCs are you are about Macs (a probably assumption) post 'em here.
-Xy