Just to keep you updated, my new hardware works, Windows 7 is up and running, too. The specs changed a bit, it's now an AMD FX-6100 (6x3.3 GHz) and a Radeon HD 7770. Man it's fast! And silent. But no more time now, I've got the Orange Box and have to play Half-Life 2 (waiting for this since years ...)
Of course it was a bit more expensive ... but less than 500 €.