Hi Dayna:
Here is how to price out point-to-point tix for your travels. You can then compare to Eurail pass and decide what to do (keep in mind you need to buy Eurail pass before you leave the US).
Barcelona-Firenze by night train (should be about 95 EUR if purchased in advance):
Go to tgveurope.com and search for travel between Barcelona and Milano (the night train usually runs on
Tues, Thur, and Sun)
Then use trenitalia.com to search Milano to Firenze
Firenze to Paris by night train (should also be about 95 EUR if purchased in advance):
use trenitalia.com and search Firenze to Paris, you should be able to purchase electronic tickets.
Paris to Barcelona by night train (should be about 70 EUR if booked in advance): use tgveurope.com and search Paris to Barcelona - one some days there are two overnight options, should be at least one every day
As long as you have a VISA or MasterCard, you should be able to book tix on these sites...to get electronic tickets just indicate you will be picking up tix in country of departure (can also paper ticket them via postal mail, but that's usually less desirable).