Each of the Memory parts you specify should operate between 303MHz - 400MHz clock rate if they are compliant to JEDEC specification. Per JEDEC all speed grades are required to operate at a max. TCK of 3.3ns (303 MHz).
Please review the AM335x data sheet. The Section 7.7.2.3 has details about DDR3 memory configurations. Possible Device configurations are listed in Table 7-57/7-58. Figure 7-47/7-48/7-49 show the schematic connectivity with supported memory topologies.
AM335x does not support 2 chip selects. So, you have to use 2 x8 512Mb devices to get a total density of 1GB. Most of the standard memory vendors do have 2 x8 512Mb parts.
2 x8 512Mb will have 1 unit load on the data bus. Using a twin die x16 part will have 2 unit loads on the data bus in addition to requiring 2 CS. So, they are not equivalent.
Regards, Siva