im pretty sure the ATX/micro ATX refers to the size of the motherboard.
I had an computer that has an microATX board, and in a small case compared to my cases nowadays.
i searched your computer and it looks like one my friends have, and the PSU looks like the size of the most.
heres a recommended PSU to buy for cheap, and its a good quality PSU too.
Antec Basiq BP500U 500W ATX12V Version 2.01 Active PFC Power Supply - Retail
its 500W, and im pretty sure thats enough for your system.
for those that knows this PSU, and say the OEM one is cheaper, but the OEM is $40 and the shipping is $10, and the retail is $50, and free shipping, so its the same price, and its better to have the retail right?