Rome to Paris Overnight Train

Once you are in Europe, how long until you take that trip? Ideally you would want to have booked it before now if you are taking it soon after arriving.

about 10 days... thanks for the fast response.

At this late date, your options may be extremely limited. Since booking tickets on the Italian rail website has been problematic for American credit card holders, I would get on the phone ASAP to RailEurope and book your ticket...that's about all that you can do as of now. For 2 July, they still show some berths available on the Artesia overnight train at $184 pp. They aren't specific, but that's probably for a couchette in a six-person compartment. You'll have to get RailEurope to send you the tickets, but you can probably get them to speed up delivery for a price.
If you don't like the price or selection, check www.whichbudget.com to see what discount airlines fly that route (several do) and whether there are any cheap fares available for your travel day. Be cautious of check-in and luggage rules.
As for Paris-Nice, you CAN get some discounts at the French national rail website, www.tgv-europe.com . Simply keep your country of origin as "Great Britain" (to keep the site in English) and look for the best fare, considering restriction on refunds and changes. These tickets will be available to print at home or pick up at any railway station.

I would also suggest maybe trying EurAide - they have a US office that can help with your reservation 781-828-2488. Once you confirm your itinerary, they can overnight your tickets within a matter of days. The cost for this service is $50. Good luck!

great, thanks everyone!

or fly, likely cheaper at this point..