Thanks everybody.
To clevelandbrowm - I agree with you but since I'm planing on doing the same trip back I'll be able to enjoy the scenery by day too.

To ssk4f - Sorry for copying your itinerary but this only shows how clever we are, isn't it? And yes, I have several questions for you but I'm not sure if I can reply specifically to you. Anyway, some questions are: The overnight train to Rome from Paris leaves at 6PM. Did you have to use 2 of your credits? I was thinking of going from Paris to either Venice or Florence instead so I'll save one of the days. Did you take the couch? Is it worth it? Did the trip feel too long? Well, that's all for now but I'm sure I'll come up with something else later.
To ira - I checked railsaver.com and the have suggested me the same pass I was thinking of buying in the first place. And I also realized I have to pay extra for a reserved sleeper. Thanks anyway.