How many GB's?
Adding an external drive certainly helps as long as you make it a habit to move files off the local drive to the external, but you're unlikely to carry around the external.
I'd be inclined to say the money's better spent on increasing the size of the local drive. It's not that hard, and there are free programs, that will allow you to 'image' your current drive, then move it's contents to a new, larger local drive. And you can swap the drive yourself in the case of most laptops, it's just like 12 screws on the back of the lappie, maybe one or two holding the old drive in place, slide it out, put the bigger/newer one in, and then put it back together. Then the same program that did the backup can be used to 'restore' the saved 'image' of your old drive ... to the new drive.
You do need some kind of external drive though to (temporarily) hold the contents of the backup of your current system (so it can be transferred to a new drive), however backups can be compressed and thus the drive used for this purpose won't necessarily need as much capacity as your current local drive, potentially MUCH less. Depends on the nature of the files you're backing up and whether they're inherently compressed such as JPG and MP3, which won't shrink any more when backing up.
BTW, 'memory' is decidedly different from 'storage space' ... 'memory' is basically temporary, extremely fast storage, and it's something your computer is using/swapping data in and out of ... every second you're on the computer, and is erased every time you power off.
Running out of memory has a decidedly different effect (and solution) vs. running out of storage space 
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