Overclocking a GPU isnt hard. It comes with software to help you do it. I found it easier than Overclocking the CPU which I assume you know how to do.
Thats a nice build, any 7970 thats under £300 wont provide any benefit over the 7950. I would suggest dropping the h100i for something cheaper, maybe a H80i or a dark knight. I would also suggest getting this 7950 instead:
MSI's overcloking software is one of the best in my opinion, ASUS' is better but ASUS does have all the nice software. I think the Dark Knight would make a very good replacement in fact I use one over a H100i in my build. Just got my H100 lying around in its box lol. Probably end up ebaying it.
I have this Windows 7 USB-DVD tool that does all the work for me. I have also downloaded the ISO which you put into the tool. That tool does everything to the USB and put Windows 7 on the USB. I have the product key if you are wondering.