I've been to Cuba 3x and absolutely love it!
Just a few tips: Do take some cash as you will probably want to leave the resort sometimes 
and may want to do some excursions
Don't bring US dollars, the only currency accepted is Pounds, Euros, Swiss Francs, Mexican Pesos and Japanese Yen.
Bring enough cash to last the holiday, can be difficult to get money at ATMs (and banks work slowly!), you can keep it in the room safe, only costs about 2 CUC a day.
Credit cards carry an 11% commission, so well worth using it only in emergencies!
Have a great time, Cuba is wonderful!
Oh yea, and have a look at the Lonely Planet forum, there's a whole section devoted to Cuba!