From what you said, you want to put the MB from alienware x51 r2 into other case, sharkoon vg4-w. Also buy the new GPU and PSU too. I think you may have problem because the back panel I/O shield ( alienware x51 r2 ) can not be removed. Here is the picture I found
http://g-ecx.images-amazon.com/images/G/01/electronics/dell/cnet_dell_ax51_gallery_11_800.jpg you should find more info online to see someone do the same. If you can solve this problem, then the PSU can fit into the sharkoon vg4-w, by the way, recommend to buy other tier 1 or 2 PSU for the gtx970, check out the PSU tier list http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html