As others have noted, the 6600 and 7700 are not long arms.
In early September I bought the 6600. Started looking at the 6500 and soon went up to the 6600 because of the AccuFeed system. I forget now what the upgrades were to the 7700, other than more fancy stitches and the larger harp. Although, there was something "missing" on the 7700 that the 6600 had.
If you're considering these, check the Janome site, and compare the specs of the two together.