You can try the online "Which" magazine for comparisons, desktop machines are being over taken by laptop sales nowadays so you should be able to get a fairly decent machine for around €500 depending on the brand, prices are constantly dropping and the specs are constantly being increased
You should be able to get something along the lines of 2.5+GHz, 1-2+GB Ram, 160+GB HDD, warranty, DVD RW etc, the main thing is to shop around. If you are using photoshop for your image editing then ideally you would want around 4GB of RAM and a decent graphics card nVidea.
You are better off aiming your specs at what you want to use the PC for rather than price and then seeing what is available within your budget. HP have a handy online Product Advisor which will give you an idea of the specs you will need: