If you are looking to spend a LOT of money (yes-250 Euros per person) you could do no wrong at Mirabelle at the Hotel Splendide Royal on the Via Veneto. The views from this restaurant are spectacular-the Villa Medici, the Pincio, the Vatican. The food is excellent as well. If 250 Euros per person seems a bit steep (and with the US dollar still weak, it certainly does seem steep) settle for a drink at the American Bar Mirabelle. There are a few tables on the terrace for patrons. The drinks won't be cheap-but definitely more affordable than dinner!
Readers: Which restaurant has the best view in Rome?