I lived in Rome from January-June 2002. I took Italian all throughout my undergraduate years, which is probably why I survived the way I did--I'm probably fluent now. Where I lived, which was Rome's equivalent of the Upper East Side, most people knew English, because they were involved with American Universities or multinational corporations--and most wanted to speak English with me all the time! I didn't want to--I just wanted to improve my Italian. I traveled throughout Italy--the Piedmonte, Sicily, Naples and the Amalfi Coast, the Veneto (Padova, Venice, and Verona) and Tuscany.
I absolutely loved Paris! My plans to spend that time with friends fell through, so I was able to spend most of the 5 days I was there doing what I wanted to do. I'm a hardcore traveler, so I had no problem doing 14 hour days walking, and the Paris Metro is awesome! I don't speak French, but I made sure I knew basic phrases which wouldn't make me look like an Ugly American.
I managed pretty well, and I wouldn't say that not knowing French would make you helpless in Paris--most things are in English, Spanish, French, and Italian. Paris has fairly decent hostels in its city center. I stayed in one very close to Pere-Lachaise, the place I wanted to visit the most.
Guides...Let's Go was great! I bought one for Rome before I moved and didn't get to half of the stuff in the book, although I tried hard. Let's Go, for those who don't know, has sections for trips of varying lengths, along with sites which aren't major attractions, but are pretty damned cool. Lonely Planet wasn't that great--I don't think they update as much as they *claim* to.
As for Eurail passes, I would wait to buy them once you're in Europe. They are flexible, but you can get cheaper tickets with somewhat better flexibility in each country. That was the experience of various friends who studied abroad--it may be different if you are spending a summer.