Firstly, before anyone else says it, there shouldn't be a danger of you losing anything because you have current backups, don't you?

. If you don't, you should.
If I understand correctly what you have and want to do, I would do the following:
- Buy external HDD
- Buy new 300GB internal drive
-
Carbon Copy Clone your current 160GB internal drive to the External drive
- Remove main internal drive that you just cloned and install new 300GB internal drive
- Copy cloned files from external drive to new 300GB internal drive.
- Done - well not all done because you should look into your backup options and schedule.
Along with your existing backups (

), you will also have the original 160GB boot drive intact but not connected to the machine.
Anyone find any problems with this?