Yes uATX is the same as mATX; they have the same dimensions: 24.4x24.4cm / 9.6x9.6".
And it looks to me like the mounting holes are in the same location as standard mATX, so it will fit in the Storm Enforcer.http://bizsupport1.austin.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c02560084
According to Diamond's website
the 7970PE53G is 11.6" long (29.5cm).
According to CM's website
the Storm Enforcer can fit graphics cards up to 27cm with the harddrive bay installed and 39cm with it removed. So you will have to remove the middle HDD cage to fit the 7970 in, but this is a simple procedure.