1.5v or 1.8v DDR2 667Mhz PC2-5300 or PC2-6400

[SKA]

Which of these three DDR2 RAM modules is better for T61/T61p boards - the 1.5V or the 1.8v and the PC2-5300 or 6400 ? I plan to get 2 x 4GB = 8GB per notebook(15.4 inch) .

4GB 200-pin PC2-5300 CL5 8c 2x256x8 DDR2-667 2Rx8 1.5V SODIMM (p/n CJN)
http://www.memoryx.com/cjn.html
OR
4GB PC2-5300 DDR2-667 CL5 2Rx8 1.8V 200-pin SDRAM SODIMM (p/n CHY)
http://www.memoryx.com/chy.html
OR
4GB PC2-6400 DDR2-800 2Rx8 200-pin SDRAM SODIMM (p/n CQE)
http://www.memoryx.com/cqe.html

Thanks
Ska