I've had no problems mounting standard boards in Gateway PC's. Even their "flex ATX" Essential series is almost big enough to hold an mATX board, so any flex board will fit. The only problem I've had with HP is that they have pressed in mounting points and come with mATX boards, so that you often have to cover the rear middle mounting point with tape to use full ATX boards without that hole.
The ones that give me the most problems are Compaq and Dell. Some IBM's use standard boards...most don't, but their tiny NetVista "flex ATX" case is actually mATX sized (four slots), which means it's even easier to find boards for that model.
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>