The point about the i7 bloomfield (i7 930/950) and i7 gulftown (i7 970/980X) is that in lightly threaded applications (like games) they are basically no faster than a £140 i5 760. This is because they are all built with the same architecture therefore are core-for-core, clock-for-clock pretty much the same. The big diffence comes in when you start using heavily multithreaded applcations (not games) - in these cases the hyperthreading of all the i7s helps to produce a performance benefit of up to 25%, and with the Gulftown chips these have 50% more cores - so they can have up to an extra 50% performance boost on top of this.
Therefore, if you are running some highly multihreaded applications (like rendering) then these i7s can provide a solid performance advatage that justifies the extra cost. However, in lightly threaded applications these gains are just not there - hence why we recommend an i5 if you plan on gaming.
Also, please bear in mind that the new Intel "Sandy Bridge" CPUs are coming in early January. These are the next generation chips based on a new architecture, but will replace the existing s1156 i3, i5 and i7 CPUs and cost about the same. These chips will be faster clock-for-clock than the existing ones and are clocked faster at stock - so in CPU heavy, lightly threaded applications (like some games, eg. BF:BC2, ARMA2, Civ 5, GTA 4 ) these will provide a performance boost.
Also, please don't buy a new 5870 now, the next generation AMD GPUs that replace the 5800 family are arriving in the week of December 13th.