Go for the latest cx42 or cx52 series if the price difference is not much. These are 2011 models with x-reality engine contrary to the previous models which sport BE3. Practically i dont know the diff. between the two when it comes to image processing. But to be safe, it is always wise to stick to the latest ones...
Apart from the above, the cx series comes with MP4/divx playability and ethernet/wifi (dongle is sony specific) support. Though it recognizes only certain codecs which come under the MP4 frame, still it is better than the old ones which equips only mpg1/2 formats...