ATX standard size is 12" x 9.6", anything smaller is mATX.

I will not get involved with mobo recommendations, but Asus and MSI seem to be the favourites. Plenty choice there.
Stay away from Jetway, ECS, Asrock etc.

 

well by the look of it looks of it, it is a MATX bard and if u r going in for a new MB and stuff u also have to get a better PSU(power supply) for that system i couldnt find any ATX board at newegg(as all the atx ones seem to be for fsb400/333 and urs is 200/266) so i think it will be diffcult to find one. if u have change then u have change the entire stuff to go ATX board and matx is not advisable as u already have one just try to change the PSU and Graphics card and u will be ok