Overlocking Wattage Increase

What sort of games do you play? Normal overclocking is fine but changing your wattage will definitely result in higher temperatures. Honestly unless you want a minimal 2FPS difference in most games I would just do a normal overclock especially on a brilliant card like a HD 5850 (Which im receiving) You can easily reach HD 5870 speeds without changing your wattage. Try MSI Afterburner which I find extremely useful

To be honest, I'm just getting back into PC gaming so I'd imagine it'll be a lot of Starcraft II and FPSs mostly. The reason I was asking is that I'm going to be overclocking the card, but I want to have a rough idea of how much power my system is going to draw. I also want to know what to expect from my overclocked card. I haven't really found a lot of information on how power consumption changes after overclocking GPUs so I was hoping that the forum would be able to help me with that. I'm a little energy conscious and while I want a beast of a machine, I also want to keep its power consumption at a reasonable level.

In researching processors I've found that power consumption increases dramatically as you hit the limits of the overclock (at least for the processors I was interested in). I'm wondering how the GPU fairs in comparison; I want to know if there's an optimal point just before the overclock gets out of hand with the power consumption or if it's a more proportional increase.