What can I use a iMac G5 17" 1.9GHz for?
It can do moderate Photoshop provided you use version CS2 or earlier. Or do a home server job.
But it's too slow for modern internet (I mean Youtube, Gmail, Facebook, any flash etc.) and it will not play any HD h.264 compressed video (even 720p) without lag.

Have a look at Geekbench Mac benchmark to have a broader picture how this one compares to other Macs