Hi diseases,
The most appropriate method is to use a tool that takes incremental backups. When you first setup, a backup image is taken of the drives you select. After that - ie whenever you tell it to run - it will only backup the files that have changed.
Vista has such a tool built in and will run from Windows Scheduler quite happily - just go to Computer --> Right Click on a Drive --> Properties --> Tools --> Backup Now... and it will take you to a screen that will search for an appropriate drive to place the backup onto and then ask what you want to backup. The down side is that if you're not using Vista Business or Ultimate then it will only backup videos, documents etc. and not executable files.
Tbh the Vista backup tool is sufficient for my needs and runs every night so I havent had much need to test other programmes.
What is your budget for the new drive? Given that it will only serve as a backup drive and you wont be using on a regular basis, I suggest just finding the largest and cheapest drive. As far as I know, I've only known there to be issues with Firewire and Vista-64, not with USB2. All this results in is a slower transfer speed. But again, this is a backup drive that you're not going to use for everyday work and will only be used for backing up and (hopefully never) restoring the backups. Speed shouldn't really be an issue as long as you set the backup to run at, say, 2am.
This 750GB drive from Dabs seems appropriate:
http://www.dabs.com/productview.aspx...,4294955542,12 and has USB, Firewire and eSATA interfaces and is £78.
Alternatively, you can probably go for 1TB and slightly cheaper from PCWorld but will be limited to USB2 interface only.
Hope that helps!
RecT