32x64 or 64x64 ddr?

[azntiger0586]

i have a question about ram. i am interested in buying 512 ddr pc3200 ram. while searching for some on newegg, some of the ram were labeled with 32x64 and 64x64, what do these mean? and which would be better? and another question is what is the difference between ECC and non-ECC? many thanks