If you already had a 3570, then there would be little point in upgrading to 4670.
But, in a new build, I would go with haswell.
If nothing else, it is 5-10% more efficient per clock.
You will have a 1150 socket that will support future broadwell cpu's.
And... the Z87 chipset has benefits, like more 6gb sata.
And... why not buy the "K" version regardless of which you pick? A 5% increase in the cpu price will bring you a 20% improvement in cpu power.
That would make the cpu part of your build last much longer, and at resale tie you will get the difference back plus more.
"it is 5-10% more efficient per clock."
but, the bottle neck in my system is the gpu and i cant justify that small performace boost for an extra £30
"future broadwell cpu's." i wont upgrade for atleast 3 years and by that time they would of changed the socket again (most likelly)
"more 6gb sata." the extreme 4 has 4 6gb sata ports - that should be plenty
"why not buy the "K" version regardless of which you pick?" this is my first pc build and, i don't want to overclock - i understand it has its advantages but, it is my 1st build and i don't feel like having the hassel of overlclocking.
In the end, worth is something only YOU can determine.
And, you are entirely correct that the graphics card is more important, and is the real engine of gaming.
My take is that a 4670K is a much longer term cpu if you consider overclocking. A conservative OC to 4.0 is trivial. A 20% boost is easy.
You do not need to do that at first, but It is a nice option to have. All you do is raise the top multiplier in the bios from 3.4 to 4.5.
As a added suggestion, use a 120gb ssd for the os and a few games. It will make everything you do feel so much quicker.
With 240gb, you may never need a hard drive at all. Consider deferring on the hard drive until you need more room.