Here are the recommended specs:http://us.blizzard.com/support/article.xml?articleId=26242&locale=en_US
512 MB of memory.
For Nvidia Gefore, 8800GTX or better; for ATI, HD 3870 or better.
I think you have a motherboard GPU, which is integrated, so you need an actual card.
You also seem to have a Micro ATX form factor motherboard, so you can only use the low profile ready graphics cards. Also what is your power supply watt power? I myself have a micro ATX form factor motherboard and I am waiting for the MSI Radeon HD 5670 or MSI R5670-PD512. At the moment you can get the Radeon Sapphire HD5570 if you have a low watt PSU. If you have higher wattage, then you can go for some of the NVIDIA low profile graphics cards.