RAM Question (2Rx16 vs. 2Rx8?)

Ok so I got a new dell vostro 1400 about 3 weeks ago. It came with 2GB of DDR2 RAM at 667Mhz. My mom just got my dad a new inspiron 1520 for christmas, some special they had on QVC and it came with 3GB of DDR2 RAM at 667Mhz. Well, this might sound bad but my dad doesn't need that much RAM as all he will be doing is using the internet, MS office, and other basic stuff. But I use lots of stuff and so I was going to swap out the 2GB stick in his and switch the capacities. So I opened them both up and when I was looking at the sticks I noticed that they are the same except that on both 1GB sticks in my vostro they said 2Rx16 on them. Both the 2GB and the 1GB in the insprion said 2Rx8 on them. Does anyone the difference in 2Rx16 and 2Rx8 DDR2 RAM? I have looked on the net but can't find any real explanations of the difference. I already went ahead and swapped the chips. Both computers run fine and windows shows that my vostro now has 3GB and the inspiron now has 2GB. In the setup page it says they are both running in dual channel mode but I still don't know what the difference is, if any other than some writing on the chips. I did notice that the 2Rx16 chips appear to have more (and larger) black modules on them.