well for starters... active means they have amps in them and therefor are powered.
i would recomend some of the behringer 2030a's or 2031a's , you could probibly pick a pair up for about 200, they have a pretty flat response and include an eq on the back for room compensation.
i would stay away from cheaper m-audio or fostex moniters,.. or any other cheeper moniters for that matter, anything below 150 dollars is probibly just a glorified pc speaker, and probibly a waist of your money.
its really not worth your time or money to buy a moniter that doesn't give a true response, remember, there is a diffrence between hi-fi speakers, witch are built to make your music SOUND good, and moniters witch are supposted to be built to let you hear what your music REALLY sounds like.
the flatter the response the better. if your monitering with speakers that hype the bass, then your probibly going to cut back on the bass alot when mixing and then when anyone else listens to your song, their going to be like.. ''wheres the bass?"
your better off saving your money and getting something good, or atleast decent.
the behringers (2030a's and the 2031a's) were reviewed by sound on sound, and a few others and were rated pretty highly. i have used them and put them along side my mackies and they quality blew me away. i havn't seen a better speaker for under 300
but thats just my $.02
good luck
-Matt