I'm looking to buy an Nvidia GeForce GTX 680 2GB for my new PC build and I'm just wondering will it make much of a difference if I buy an £360 EVGA card or an MSI twin Frozr which is £400 (i know an extra fan but in general does the difference between different brands make much of a difference i.e. does it affect OC potentially) and also is the boost from OC'ing worth the hassle?
Your asking two seperate questions here. One your asking if the brand makes a difference and two your asking if the aftermarket cooling makes a difference.
As long as it is a quality brand, it is mostly up to your personal preference and which has the best warranty.
The first card your asking about is called a reference card it is all stock and comes equipped with a blower style fan. The advantage of this is it blows all the heat out the back of the pc instead of recycling it back into the case.
The second card you posted is what is called an aftermarket card. This means the company making the card has made its own changes to the cards heatsink and fan setup, PCB, or the connectors on the back. Aftermarket cards are usually factory overclocked and built from better quality components leading to better overclockability. Sometimes reference cards can be factory overclocked as well usually not as high though.
By having better cooling on the video card it will be quieter, run cooler, and generally be more overclockable. I prefer aftermarket cards myself the noise and temperature difference is sometimes significant.
Here is my favorite 680 (I believe it is the fastest as well) http://www.newegg.com/Product/Product.aspx?Item=N82E168...
I have one and it is very very quiet. However the big downside is that it is a triple slot design rather than double slot. If you can make it fit then it is what I would recommend.
Overclocking is so easy to do now a days that it is definitely worth the "hassle"