You'll need to provide more information about your laptop: brand, model, etc.
In general with older machines, if you can upgrade the amount of RAM, you'll tend to get the best bang for your dollar (or pound). The first thing you need to do is find out how much RAM is currently installed on your laptop, then determine if you can add more. In other words, if you're already maxed out, there's no possibility of upgrading.
For example, I've successfully purchased used ThinkPad T420's and upgraded them from 4GB to 8GB of memory. The Crucial site makes it easy to look up compatible memory cards for your specific laptop:
http://uk.crucial.com/gbr/en/ (UK version). I first look up the version that I need, then attempt to order it from ebay or amazon.
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Probably more technical than you want, but another alternative would be to install a free minimal Linux operating system on your laptop, e.g. Xubuntu or Mint. I run Linux on my work and personal machines. Even on the older equipment, everyone is surprised at how peppy it runs without all the crapware running in the background.