Dual Channel Memory

Quick question...

I'm looking to purchase Mushkin 4GB (2 x 2GB) DDR2 667 (PC2 5300) memory for my Macbook Pro. This is stated to be dual-channel memory, my question is later on when 4GB memory prices drop, will I be able to remove one of these 2GB chips and install a 4GB without problem or does the dual channel imply that each chip must be 2GB. (I understand what dual-channel means the same chip sizes yet wasn't sure if they completely wouldn't work without equal chip pairings.)

Also is a CAS from 5 to 4 worth $10 for someone who uses photoshop, vmware somewhat frequently?

Thanks for the help in advance.

 

I've researched the dual channel technology a bit more and apparently you can have different sized chips with dual channel based chips.

Any advice towards the CAS latency difference though?