I ordered a ticket for the Borghese yesterday from the USA using the named website. It was fairly straightforward, but my B of A credit card didn't work. Some banks require prior notification even for this sort of transaction since the sale occurs in another country. Anyway, I had another credit card that worked easily. I will pick up my ticket at the box office. The total charge was 17 Euro.
I got a ticket for the day I wanted, but not the desired time. I recommend making that reservation more than a week or so in advance if you want to be sure of getting the entry time you want. And take a printout of the reservation with reference number.
In 2010 I visited the Borghese. When I picked up my ticket half an hour before the indicated time I was let in to the gallery right away. So, I had more than 2 hours there if I wanted it.
I plan to take a bus from Termini to get to the Borghese. The easiest way for me would be to take a taxi up and maybe walk back down to my hotel. I'll see how my legs feel. The gallery is located within a beautiful, green park which is worth a leisurely visit.