Despite what the eBay sticker says about GDDR3, I looked at the zoomed pictures and based on the chip markings (j1108bfbg-dj-f), those chips are DDR3-1333, so no GDDR3 weirdness here aside from the misleading sticker. Whatever issue you have getting the extra DIMM to work is either vendor whitelist or compatibility issue.
As for putting Windows on it, that would depend on whether the device as a standard PC-like boot interface - legacy or UEFI - and the ability to boot off external storage.