Here's an up-to-date guide for computer parts and their price ranges in US/CDN currency. Use currency exchange and all your other factors that are applicable to where you live.

http://img833.imageshack.us/img833/7372/guideu.png

As for Archangel's predicament, I strongly suggest that you ask a local or regional forum in the country of your residence for consumer sales. Otherwise, if you're strictly looking at a "decent" pre-built computer (Microsoft Office, media and some "gaming"), you're probably looking at something like this that will last you at least five years:

HDD: 1 TB (5400 RPM usually default).
RAM: 4-6 GB DDR3 (bus frequencies shouldn't matter at this point).
Case: Not too important, but the bigger it is, the more fans you can put in for cooling and is easier to upgrade should you want to do so; make sure it dissipates heat well, though, that's very important (either water cooler or multiple fans).
PSU: 550 W+, with as many fans as you can and see if it's energy efficient of at least 80%+ for a smaller electric bill.
GPU/Video Card: Most older games (pre 2003?) can play perfectly fine on your chipset and often, the pre-built computers don't come with a video card, so you'll have to buy it separately yourself (512 MB is the minima these days).
Socket: AM3 for now...
CPU: You're probably looking at an Intel i3 or i5 processor.
Motherboard: Shouldn't matter what it is, but you should know what kind of specs it supports if you would want to upgrade your parts in the future to make the best out of your motherboard.

By pure currency conclusion, you're looking to spend about 500 Euros without the graphics card. From the looks of Europe and their economic policies, that might be somewhere in the mid 600 or low 700 Euros for the same computer in the US.