What external HD?
 

I'm looking at getting an external HD to use as a back-up for my laptop.

Can anyone recommend anything that is reasonably priced with about 80GB storage?

How do these connect to the laptop? Is it via a USB port?

Yep....**Technical numpty alert!** :D :D :D

For a laptop, I'd go for 2.5" usb drive so no additional power required.
I like the buffalo, Lacie have some ruggedised ones, western digital have a funky black one. Stick with the "known" makes with decent warranties and you should be ok.
Any of the main retailers will have loads but dabs is probably easiest to navigate to see the options and prices.
Some are supplied with a dual usb cable in case a single laptop one doesn't provide enough juice, this is a feature I now look out for.

I have a couple of external drives. I purchased the drive enclosures and disks separatley so I could spec the capacity of the drive I put in there. At present it's two 320GB Seatgate drives, so that should cope with my storage requirements.

Given that drives aren't too expensive you might want to consider a similar approach, all you need to do is get an enclosure (3.5" external usb enclosure is what they are usually called) and then decide on what size drive you want. I'd tend to go for the largest you can afford as it's always better to have more space for available for when you need it than to run out.