Memory question

[pdudas]

I wanted to buy memory modules for my X60. Found that I have to buy Pc5300 DDR2 667MHz CL5 Sodimm modules (like the original 40Y8402/40Y7733 modules was in my thinkpad).

On the following Lenovo page I found the memory modules for X60 systems:

http://www-307.ibm.com/pc/support/site. ... MIGR-63277

First question:
How can we use PC4200 modules in a 667mhz memory speed notebook?

2nd question: will the dual channel work with different size modules (1Gb+512Mb) or I have to buy 2 modules which are the same size?