Personally, I'd recommened you get a hold of the latest issue of PC Gamer. They always have a good hardware section, as well as all the techy names which I forget.
They have a 'PC gamer Rig' which is updated monthly, and that costs under 1000 pounds to build (yes I know euros are less than pounds), this months was just a shade over 900 I think, and thats from the ground up.
In short: Quad Core, Two Direct X 11 graphics capable cards, and a healthy amount of RAM (several GB) as well as Windows 7 (safe bet, dunno if they'll support XP, its over 10 years old), maybe a SSD (Solid State Drive, a faster type of Hard drive)
If you order the parts seperatly and find a put-them-together-yourself guide, you might be able to skimp on the Mottherboard (I recommend an SLI capable one), convential Hard Drives if you've already got them, mouse and keyboard if your happy. I would recommened a nice monitor, however, it makes a surprising difference
That wa, you'll also cut on service charges that website probably charges.