Yeah most games dont push multicores to the max yet, but we're slowly getting there. Faster thanks to next gen consoles I hope.
I'd definately pass that offer with your setup
definately go with 7870, in your case if you want to get up to double the longevity out of your setup. I think Sapphire or HiS has some good models going for 150-180£. If you look around a bit you might find some for even less. 7850 is priced in a way that its just not worth really going for it over the 7870.
http://gpuboss.com/gpus/Radeon-HD-7870-vs-Radeon-HD-7790 some comparisons