The £7-800 range is basically in the middle of the decision, £150 eitherway and it'd be a no-brainer.
Looking at the mesh site, some computers on there have a quad core, but pitiful graphics cards, for the money.
And why so many of their computers have 4Gb of RAM is beyond me, 2Gb and a better graphics card would be a much better PC than 4Gb with a 8400GS, can't understand that.
The quad core has a definate performance advantage in benchmarks, but not one that applications are fully utilizing at the moment. The thing is, by the time quad core is a utilized by most games/applications, much better quad core CPU's will be out. The E6750 isn't going to be obsolete overnight either.
My advice would to either go for the E6750 or E6550 and spend the money elsewhere on the PC, or spend an extra £100 or so and get the quad core with a decent graphics card etc.
Quad core is definitely better, and a year or 2 down the line will start to show it's teeth, but don't get one by compromising the rest of your computer.