I'm no expert, however I looked at the recommended requirements for Diablo 3 and the computer you've chosen, looks like it has met those requirements (considering you can afford windows 7 and a hard drive). If you want to have a look your self here is the link. http://eu.battle.net/support/en/article/diablo-iii-system-requirements
However, I would recommend looking around on this website, I heard they are pretty good. http://www.chillblast.com/Gaming-Systems/
Or, if you want to save a bit of money you can build the system yourself.
Hope I helped.