Prehaps you should have also mentioned when overclocking IF you don't up the game in regards to high performance cooling you will hit a big fat dead end real soon, as for a 20-30% overclock which certainly IS in the realms of a safe-ish overclock I very much doubt it would add 30fps on games that are running under 100fps BEFORE overclocking.
The only scenario for 30fps gains to happen would be a rig where it has a seriously slow cpu running (say dual core 2ghz) on a mid to high end motherboard with a gpu like a nvidia 280gtx or Ati 5870 (likelyhood of that? very low chance) it just wouldn't work out in the maths otherwise, then it soon becomes obvious that is there a point if the games at 100fps when most monitors are 60fps capped?
Anyway this brings me back to my statement of 5-10fps boost being realistic on a modern game when overclocking, hell my own experiments showed that 66.6% overclock gained virtually nothing on GTA4 (The game never even uses 50% CPU before the overclocking) for example, now had I a better GPU it may have certianly jumped up 30fps but a better GPU would have similar effect without overclocking. Therefore to overclock you need a rig thats ALREADY of decent spec to make it worth the hassles or the gains will be poor, as more and more games rely on the GPU to do much of the hard work, sure without a juicy CPU it will bottleneck, but it's important not to give people false hope of huge gains if they have an average GPU & CPU Combo.