What do you want it to do. I mean, most computers will do for most things that people use nowdays. nearly gone are the days when you needed to really customise it to the activity.
So it comes down to what you want. Whether you want it to run two monitors, have an HDMI output for the tv, be silent, have two hard disks for data backup, have usb3.0 drives or firewire, need to have blu-ray, want to play xbox type games etc etc..
You do a lot of photography don't you? So a good graphics card (that's most now), a good chunk of memory, a card reader slot, that kind of thing is what you want. Think about those kind of details.
Maybe speccing it yourself would be better, at somewhere like this
http://www.chillblast.com/
I'd also really think if you need a desktop. A laptop with a docking port might be more flexible and again, motherboards and processors are changing so fast that the advantage of upgradeability of a desktop is only useful if you intend to actually upgrade it. With sata and usb peripherals are less of a problem than when you had to stick scsi cards in to run scanners and discs.