the xv645u is an 2009 model. Toshiba updated the processing engine from 4G to 5G and it also has 14 bit video processing with a 10 - bit panel. I am not sure what else has changed but you can always compare the two sets. I ordered the 40XV645U yesterday from amazon....heard some good things about the set, plus the price is nice.
I would suggest looking at their website and see whats different besides to older and newer version besides what I mentioned if there is any more.