Great info mate! Cheers!
Is it easy to overclock the CPU/GPU? Bear with me, I'm a hardware virgin, for the most part.
Quite easy yes. If you download the drivers from ATI rather than using boot camp you should be able to up the speed somewhat in the official drivers. I also believe under the normal ATI drivers they jump back up to their standard clocks anyway. But really you want the software to up the fan speeds slightly on the iMac just to keep the heat down a bit. Software a lot use is macfan, basically allows you to set the fan speed to whatever you want. Normally they run at around 1/2 or less of there top speeds. But a lot of people whack them up to around 3/4 or full when gaming to keep the temps down also gives them a bigger overclock.
There are threads on the various mac forums with lots of details and how to's etc so worth having a look around if you ever want to try some games.
With regards to the CPU though, no point overclocking it. More than up to the job plus in gaming you get very little gains with CPU overclocks. Mostly all about the GPU.
You really need to get on the forums, tried having a look for the threads but there are so many of them and lot more posts now than when I last looked. But there are various programs some OSX based, some Windows based. So you can get the performance back under both OS's. As with steam now being on OSX makes sense to have the ability in OSX too.
There are guides and lots of feedback from people who have done it, settings you should try etc. So worth having a read of it all. Lots of info on the mac forums.