It will be nightmarish traffic when you are there but as long as you know that, then you won't be surprised (well, you will be if for no other reason than the nature of drive itself and what you encounter with buses and having to back up to let them pass, etc) but if you are prepared, then it may prove a convenience. It will still be high season then.
I use Autoeurope to book cars. You might check with them. If you need an automatic, getting at the airport is likely the best bet as most rentals are manuals.
Do get an International Driver's Permit - it is legally required in Italy; you get them at AAA offices.
Oh - and read up on ZTLs (the restricted zones in the cities/towns/villages) so you don't get caught out entering without thinking and ending up with a big fine arriving in your mail.
Here's a bit of info but googling it and other driving rules, etc in Italy is needed so you know what to look out for:
(not just about Rome; has photos/info about the ZTLs and few other driving issues to know)
Edited: 1:54 pm, May 28, 2013