ddr2 sdram dual rank

hi,

 i am going to grab 2 more 1gb stcks of ram 800mhz. i just recentley bought 2x1gb 800mhz to move up from 533mhz. so the question i have is. the first 2 modules i got i had seen in the specs that they are 128x64 is that the same terminology as dual rank or what is dual rank.  dell sold me those as a paired set but they r sold individually serial numbers are way different.

 ok so im looking at a dell 2x1gb kit and there is like 3 kits of the same specs listed which makes no sense to me. they also have different manufacturing part numbers and dell part numbers but there the same but they are listed as dual rank in the kits..  my last too where dell but kingstons with dell sticker? any help please.

xps 400/9150