"how powerful do these little mini atx builds get?."
Please, read the topic Scott...
The smallest, most powerful mITX board (which is what I assume Scott is on about) would be a toss up between the DFI LanParty H55 and the Zotac H55-ITX-A-E, both of which are socket 1156, with enough space for up to 4Gb DDR3 RAM, these are generally considered SFF boards.
As for mATX (which is what you're on about) there are a crapload with SLi, Crossfire, Socket 1366, capabilities of up to 24Gb DDR3 RAM , so pretty damn powerful, though these are considered less as SFF and more as compact gaming motherboards.