Felicity,
[I realize this topic isn't terribly new, but hopefully you haven't left on your trip yet.]
Death Valley hot. "But it's a DRY heat..." LOL
We went to Ostia in August - in our case, the 'quick trip' from Rome to Ostia took seemingly forever because everybody was riding the train to the beach; I think we stopped approx. every 17 feet to pick someone up...I was the only female not wearing a bikini (hallelujah!). Anyhoo, Ostia was definitely worth the trip - it was fun trying to imagine life there. And it's a manageable-sized site.
Wear a wide-brimmed hat and sunscreen, and bring lots of water and food/snacks. The museum was closed the day we visited; bummer, but that's Italy! No one at the site knew why it was closed...but that's Italy!
Go to Ostia instead of Pompeii/Herculaneum if that fits your itinerary better. If it's any comfort, comparing these ruins (or churches, 'quaint villages', etc.) is like comparing 1ct and 2ct diamonds. Ummm, nothing wrong with choosing the 1ct! LOL Still trying to get to Pompeii/Herc. though...
Have a great time, and go slow and steady in the August heat!