You've got a 2500+ same as me. It runs everything I can throw at it and then some. Why do you want to overclock it, if you overdo it do you have a spare £100-£200 lying about to pay for the replacement parts? I really don't recommend it for what thats worth but of course the choice is yours.
No matter how much you overclock, the real difference is not worth it. You might be able to prove it is running faster using synthetic benchmarks such as 3D Mark etc but in real terms it won't make a toss of difference to your PC and you could end up with a pile of molten silicon under your desk or worse.
My advice if you are not happy with the performance of your PC is to stick as much GOOD QUALITY RAM in as possible you won't regret it.
By good quality I mean Corsair or Crucial etc. and I would highly recommend a couple of sticks of 1024Mb, that will improve performance all but if you are still determined to overclock then you should be armed with the correct info, click here to start with and click here and click here to check with the experts.