Dude! Haldi's link totally does it! I fixed the link here:
Hi Rez Screenshots
I didn't even think to try searching these forums for that. lol
Just use that method, Markus. You won't have to build a crazy computer, other than getting a decent Nvidia video card and you can take screenshots at any resolution you want! I'd guess the best method is to use a VNC client as mentioned so you can remote view your picture taking PC from another; the VNC will scale the image to something you can use. Then you just use the "force resolution" in the nvidia control panel to do some crazy resolution like 4096x2560 or higher, as described in the article linked.
Nice one, Haldi!