I've done some research and found out that motherboards on the Dell Optiplex series are "BTX".
I've also found this case on eBay which claims it is a "Micro BTX Tower Case"
Link: http://goo.gl/xKrcs2
I've also found this case on eBay which claims it is a "Micro BTX Tower Case"
Link: http://goo.gl/xKrcs2