I would definitely get an amp or you are going to be chained to your room. Unless you have a lap top and want to pack that and your monitors up and move everything around.

Jeff is leaning toward amp models & monitors for himself these days, but I would bet my paycheque against his that he already owns at least one amp, right? :)

IMO get an amp, but like Jeff says, that is just one man's opinion.

Oh, and if you really only plan in playing at "bedroom volumes" you can skip the 100 watt Marshalls and whatever. Go for something 2 - 5 watts. It'll still be too loud. I'm partial to the Fender Vibro Champ myself. ;)
http://www.fender.com/products/search.php?partno=2331000000