It depends on what you do with your machine really.
You have chosen a hobby which needs a little bit of "Oomph" if you like, but not as much as some of today's games.
A desktop around the the 500-600 mark would be about right, and a decent photo-editing suite.
Monitors are also important to view your pictures so:
And a good software package to edit, focus and enhance your pictures:
Hope it gets you started :))