UH, well it's HOT - you will "perspire." As Kent has posted, make sure you have a hotel with AC. Also, carry a small bottle for water. The fountains in Rome act as "water fountains" for everyone and the water tastes great! You'll see folks filling up their water bottles at many fountains or water "faucets."
I'm not sure what to relate it to. Near the sea, with the wind it can be very pleasant, but I've also had hot (and I mean hot) days in July! Dress lightly, wear your sunscreen, drink lots of water, and you'll be fine.
To me, and I'll defer to Claire who has lived in Rome and now up north in Bergamo, August IS MUCH hotter than the period 6/26 - 7/11... but it could be because we're all so tired of the heat by August... but then, Nope - it's really hot in August!