A couple of other notes and amplifications -- the yahoo answer that I cite above makes the determination that since the Aspire x1700 has a NVIDIA MCP73PV motherboard and since gateway also has a NVIDIA MCP73PV motherboard that _happens_ to be micro-ATX, the Aspire's is micro-ATX as well.
Frankly, in my opinion this is too tenuous of proof -- I can't seem to find any other information on the public web about this and the Acer documentation seems loathe to say what the form factor of the motherboard actually is. I would imagine that Acer could have put the MCP73PV set-up in any form-factor it wants.
For example, this site (http://www.digiconcepts.com/msi_motherboards_164.htm)
shows the same chipset in an ATX form-factor. What would I do? I would get in touch with Acer and see if they'll just tell you the size of the board.
Additionally, some documentation from Acer (https://support.acer.com/acerpanam/desktop/0000/Acer/AspireX1700/AspireX1700sp2.shtml)
suggests that this unit has a 220watt power supply. I would question whether a power supply of this size could power a third-party motherboard, particularly since the P67 version of the new Sandy Bridge motherboards do not have on-board video -- you'd have to get the H67 version of the SB motherboards, which do offer an on-board video interface.