Yes you're right.
Well, imho 1640 should be worth successor of 1620, since they're based on same chipset. Besides it showed on -R format a much better performance then 1620...and if you search a bit into various tread you'll see what burns 1620 is capable of.
Another main point of 1640 is overspeeding (officially still not supported but it looks like next firmware will add it) feature. You can burn 8x media at 12x and still get excellent results... you could also burn at 16x but it's not recommended.
Add aslo thet Plextor - King of Quality - chose an OEM Benq for new PX740, and i don't think i's just a coincidence
If i should pick up a new drive now i'd be undecided between 1640 and Pioneer 110 [that will be out in 1 month].
Just my 2 cent