you could use that motherboard/CPU in a larger case, and then later on if you wanted to get a different motherboard/CPU, you would be able to use that new case with it as well. just keep in mind that the DVD burner in that Acer may or may not be able to be swapped over to the new case. granted, new DVD burners are pretty cheap, but that's another expense.
also, you might want to double-check that size of the motherboard. "mini ATX" apparently exists, but not very common. your current PSU is "mini ITX" sized... and from what i've seen online for your motherboard, it is "micro ATX". a micro ATX board will fit fine in a standard sized case
try these links that i found for some help:
http://www.geekstogo.com/forum/topic/313599-ax1420g-u5832-case-swap/
http://www.geekstogo.com/forum/topic/308458-acer-ax1420g-u5832-psu-graphics-upgrade/
http://www.tomshardware.com/forum/352987-33-acer-ax1420g-u5832
http://www.se7ensins.com/forums/threads/acer-ax1420g-u5832-case-psu-video-card-first-time-builder.733719/
http://www.findlaptopdriver.com/motherboard-ax1420g-u5832-specs/