The value of raid-1 and it's variants like raid-5 for protecting data is that you can recover from a hard drive failure quickly.
It is for servers that can't afford any down time.
Recovery from a hard drive failure is just moments.
Fortunately hard drives do not fail often.
Mean time to failure is claimed to be on the order of 1,000,000 hours.(100 years)
Raid-1 does not protect you from other types of losses such as viruses,
software errors,raid controller failure, operator error, or fire...etc.
For that, you need EXTERNAL backup.
If you have external backup, and can afford some recovery time, then you don't need raid-1.
Since you are planning on external backup, I think I would go for a single 1tb drive. It will be faster and cheaper.
I would consider a good SSD for the OS and frequently used applications. It takes about 15gb for Windows 7, so plan the rest accordingly.
Instead of using a USB attached external backup, I use a mobile rack for backup like this: http://www.newegg.com/Product/Product.aspx?Item=N82E16817993031&cm_re=mobile_rack-_-17-993-031-_-Product
It has a power switch which I leave off unless I am doing a backup. I get full sata speeds. The tray is removeable, but I don't think it is suitable for frequent changes/