a while ago i saw a thread not in english showing hp z800 with two msi gpus in sli but i can't seem to find the link but i have found this on ebay the guy showing gtx590 in sli not sure maybe he is showing how the hp z800 can fit long gpus anyways here is the link
http://www.ebay.com/itm/CUSTOM-HP-Z8...t_12129wt_1026