As Roberto says: I would compare it to Hell.
And I'd agree. Living in Rome for a few years we DID NOT have AC in our apartment... so come August, we would do like many Romans - LEAVE TOWN!
It was just too darn hot. We'd go to the mountains, the beach (Sardinia), the Lake (Bracciano) ... or to places like Copenhagen. We left Rome in August one year (102' F) and arrived in CPH (72' F)... and decided, yes... we can certainly live here. (Of course, after we did move to CPH we found out that 72' F was the HIGH TEMP for the year!).
So expect to be hot, sweaty, tired, dehydrated, and irritable (Well, the heat was always my excuse in August!). Drink lots of water from the nasoni, take 3 hour lunches in the heat of the day, and stay out later in the cooler evenings. Sounds kinda Roman doesn't it?
After some time, you do adapt to the heat; but for most folks on vacation... there's not enough time or desire to adapt... and there's just so much to see and do! So pick the hottest place you've ever been.. and then plan on climbing, standing, walking, waiting in line, all in the hot, hot sun... and you've got Rome in August! (That all said, I'd still go... )