128M x 64 VS 256M x64 Module Configuration, Whats the dif?

I am trying to hunt down some 2GB sticks of 667Mhz sodimm memory for my Dell notebooks, if you go to this page here on the supertalent website, you can see there are 2 different 2gb sticks. Everything on the 2 different sticks looks identical except for the Module Configuration. Is it going to matter which one I buy? Will the notebook know the difference between the two? If I go to dells website, here is the stick dell would sell me(check out that price LMFAOROFL). Notice on the dell website it says Logical Type 256x64. So I think the obvious thing for me to do is find some 2GB sticks that are 256x64 rather then 128x64. I'm still curious what is the difference though. If I ended up with some 128x64 modules would they not even work in my notebook?