If you want a hard drive primarily for backup then i'd say get an 80gb drive. Even though this is twice as big as your current one it means that if you ever upgrade your 40gb one to something larger then you dont need to buy another drive for backup if you see what i mean.
Here is a 40gb drive click here
And here is an 80gb drive
I'd go with the 80gb one as it only costs £4 more.And if backup is al you need it for then it will do fine.