RAM is measured in MB...well, it is becoming quite common to see RAM measured in GB, but i certainly hope you are not trying to put 512 GB of ram into a G3.
As far as issues with DVD playback...the only problem i can see is choppy playback when other programs are running.
GPS...as long as you can find a Mac compatible unit there should not be a problem. its not that they dont exist, but your choices may be limited somewhat.
MP3 should be fine.
And for OS X....basically...slow. just like any computer running anything, the older the components, the slower its going to be. if you are asking if its compatible, yes, G3's are. but i wouldnt be happy running 233 MHz having worked in GHz for the past couple years. but if you dont mind, itll work
minimum requirements for OS X:
Macintosh computer with a PowerPC G3, G4 or G5 processor
256MB of RAM
3GB of available hard disk space (4GB if you install the developer tools)