Well depends how much memory you need obviously. You can get external hard drives with anything from 50 GB to 1000GB. Your laptop probably has a 40-60 GB hard drive, so that should give you some indication of what size hard drive to get.
And of course it depends on how much money you are willing to fork out. I've quickly looked on websites, pcworld looks like it has some good deals, it has a toshiba 250 gig for 90 quid. The price also depends on the bulkiness of it, so if you don't mind not paying so much but it taking up more space, then you could afford more memory.