All mATX boards should be the same size with lined up screws.
You don't happen to have an image of the inside of your case do you? I also would not mind seeing the power supply label
I do not want to put you off of the idea of computer upgrades, but you will almost certainly have to reinstall windows(mass hardware change). And OEM versions may require you to call MS and let them know that you HAD to chance the board(lets just say it failed). OEM copies of windows ARE locked to one board. see the little issue here(most times MS gives no issues, but for legal reasons they CAN refuse.).
Your COA key should be on the case AND you can get an ISO to make a copy(DVD or flash drive with the right tools) of windows here.
http://www.w7forums.com/official-windows-7-sp1-iso-image-downloads-t12325.html
I want you to know what to expect.