Grab the RAM unless you're dissatisfied with your current cooling solution/achievable overclocks...(4.5 GHz is standard for most 2500Ks)
If you've got about 100-150 more quid to spend, consider upgrading your graphics card. (It's still a very respectable card for its age though.)
Or do what Golden says, and splurge on a new SSD when you've saved enough quid. Most forumites agree that the SSD is the best upgrade you could add to your system if you have the quid for it.