Yeah I made the mistake to trust the testing build too much. While it worked fine as an upgrade on my gaming PC, it didn't work at all on a new installation on my W520 laptop. So as I'm standing, the stable 64-bit bit 6.0 (although the version shouldn't matter, because Debian stable is quaranteed stable at any time), is the standard solution to get a computer working properly.
And regarding KDE, it seems Debian relies on GNOME on the default installation media 1. KDE might have looked more like XP, but GNOME just takes less resources, so I would say, use GNOME. You can configure GNOME to look like KDE anyway, and GNOME brings also all the good and fast tools like gparted.
I know Linus Torvalds recommends KDE, but as he hates all the kernel bloating and ext3 and ext4, I would say that maybe he should revise his recommendation on GNOME too