You should check out each operator's website for prepaid offers. As far as offers stand, Cosmote has the best offers for money value. Unfortunately, I have not been a client of Cosmote services, prepaid or postpaid, so I cannot tell you the quality.

I am however a client of Orange Romania and their services are good. A bit more expensive than Cosmote, but still cheaper than Vodafone, for example. Your N900 will automatically configure itself with the Internet settings when you insert the SIM.

Also, if you want to call home, be sure to check websites for international calling options because you get better tariffs that way.

I also attached the websites of Romanian mobile operators:

Cosmote Romania
Orange Romania
Vodafone Romania