The 7670M is a rebranded 6650M. That probably doesn't mean much to you either, but just trust me when I say it's by far the slowest of the three. The 7730M is about 30% faster than the 7670M. The XPS 15 easily has the fastest of the three, due to the fact that its GT 640M is equipped with GDDR5, vs the other's DDR3 memory.
But the XPS 15's GPU speed alone is not worth £500. Of the three, I'd take the 7730M machine, unless the XPS 15's overall package has sold you on the price, as it is a very nice product.
There is another option, similarly priced to the 7730M equipped Inspiron, but with a faster GPU than all three of those.
I speak of this machine. Configured with: i7-3610QM, 4GB 1333MHz RAM, and a 500GB WD Scorpio Black 7200RPM HDD, you're at £772.00 inc VAT, and this is with a GT 650M GDDR5, which soundly bests all of the above .