It's pretty easy to transition. I love the OS X look and fell. Much better than Windows in my opinion.
For Office, surfing, and email, the G4 processor in the Mac Mini will be just fine. For photo editing, well, that's another story. Basic photo editing should be fine, but if you're doing high-end Photoshop stuff or doing multiple images, then you might run into a problem if you are under time constraints. (The Mac Mini will handle it, albeit slower than a G5 processor, which is a 64-bit processor.)
You'll have to upgrade the memory. There is only *one* memory slot, so if you buy directly from Apple's online education webstore, you should probably order the amount of memory that you think you'll need (I suggest at least 512 MB, but OS X seems to run best with 768 MB - 1 GB of memory). Before you do this, however, make sure you check Crucial.com and a few other memory websites to see if it will be cheaper to order from Apple (and thus not have to discard the 256 MB chip that comes with the system), or whether it will be cheaper to just throw away (or eBay) the 256 chip and order a 1 gig chip yourself.
Be careful when opening the Mac Mini. If you're overly aggressive, you'll warp the aluminum case.