Yes, you do. There's a pair of BeoLab 4500s on ebay.com right now
They will do a fantastic job. Alternatively, the other options include BeoLab 5000, BeoLab 4000, BeoLab 6000, BeoLab 8000, and BeoLab Penta (mk2 and 3). I wouldn't really want to go with BeoLab 1s, 3s, 9s or 5s as I feel they wouldn't match the CX100 as a centre channel very well.
You would also then want a pair of rear speakers. As the AV7000 makes light use of the rear speakers in comparison to true Dolby Digital systems, a pair of BeoLab 4s would work perfectly as rears, and they won't break the bank!
Make sure you don't get any BeoVoxes though, as the only channel they will work on is the centre channel, unless you purchase BeoLab 150/200 amplifiers (which can be very expensive!).