Depends what you want to do really and how much you want to spend.
As far as I can tell you have a 22" screen, assuming 1680x1050, a 7970, really a 6970-50 would be overkill, I wouldn't really go over, right now, a 6870 or 1gb 560ti for the £150 mark.
If you wanted to get a 24" screen then some slightly faster cards might become more worthwhile, they might not.
I wouldn't pair a 6950 up with a dual core, full stop, but a heavily overclocked dual core (which is easy to do) is still a more than viable option for many games, only a few games will seriously struggle without quad core, most won't, not optimal performance but not bad.
Again its a case of what you want to spend.
If you want to do a full upgrade, a £150 24" screen, a £200 gpu, and a second hand Q6600 would probably work pretty well.
A q6600 will do pretty damn well in most games, again really only if overclocked, but most do 3.3-3.5Ghz easily and are pretty damn cheap second hand.
if going new and higher end, you'd want to realistically either go AM3+ mobo and a phenom 2 quad core for as cheap as possible(some of the dual cores for cheaper unlock to quad cores to save even more cash), or if you go Intel there really isn't much point buying anything below a 2500k at £155.... and AMD quad core IS much slower but it's way more than enough for gaming, and you'd be looking at closer to £80.
There is frankly a lot of options depending on your budget and where you want to go.
ultimately, most on gpu least you can get away with on cpu/mobo tends to work best for gaming as GPU is easily the most important. however again at what is now a lower res, a top end GPU will be a waste.
If money is tight, I'd consider a 560ti/6870 for now, then when you can upgrade the screen, those gpu's will still do 1920x1080 fine, just maybe without AA all the time and occasionally lower settings. Then in the future you can add in a new cpu/mobo when games really start to need a bit more CPU power.
