I went from a highly overclocked 460 to a 480 and to be completely honnest, the difference is not massive. Saying that, my 460 reached 950core versus just 750 for the 480. 480 temps go to high above 750, but the 460 stayed nice and cool.
When both are overclocked, there is not a massive difference between the 460 and 470. If you have good case cooling and a decent power supply the 470 is a good option, but the 460 is great card.
I personally would go for the Gigabyte 460 1GB SOC for under £160 because it offers 470 like performance with less noise and lower power/cooling requirements. My 460 was fine at 1920x1200 and I kind of wish I kept it and didn't bother with the 480.
edit: Your XPS 8100 case does not provide great cooling so I would definately avoid the 470. The PSU may also be a limitation for high end graphics cards. I hope you do not have the stock Dell 350W version because even a 460 may not run on this. You probably need a 500W PSU for the 460 and 600W+ for a 470.