I have 1 VGA socket on my laptop, and currently have that in use by a large screen. At some point in the future I would like to get another one of these screens and use both screens in dual screen mode but not use my laptop display (to be honest my laptop display is pretty rubbish as its like 2/3s the size of my screen - even if I had the choice to use all 3 I probably wouldn't want to). Is it possible to achieve this? If so, what do I need by way of hardware / software, and how much do you reckon it should cost me?
It depends on the capabilities of your laptop. While some laptops do indeed provide multiple video outputs (VGA, DVI, HDMI, S-Video, etc) they may either let you use more than one at once in a multiple monitor setup, or it might allow you to use multiple outputs, but the non-VGA outputs only clone what the the VGA output sends out, or they may only allow you to use one output at a time. Those are the three possibilities for built in capabilities.
The other options are external hardware. The quickest and easiest is to get a USB based display output such as this one. Beyond that there are other possibilities ranging up into the thousands of dollars depending on how fancy you want to get and what kinds of features you want.