Dell Inspiron / Latitude memory upgrade question

Hi guys,

I have two laptops - Dell Inspiron n3010 and Dell Latitude E6410 both with 4Gb memory (2 dimm each)

I am willing to upgrade their memory to at least 8Gb and maybe more in cost effective way.

My questions are:

1. Can I purchase 8Gb single dimm and put instead of existing 2 dimms, instead of buying 2x4Gb? (this way I can save space for additional 8gb dimm in future).

2. Can I add existing 2Gb dimm to the newly purchased 8Gb (to gain additional 2gb memory)?

3. Can I buy dimms with higher speed (e.g. 1600Mhz) and it will work correctly on the supported 1333Mhz?

4. Is there any difference for the upper questions between two of my laptops?

My questions are specific for those models and not theoretical. 

Many thanks

Meir