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Trying to think if it's worth it..... The 480 is less power hungry which is ideal for my 450W PSU
Also to add to that, for the 780 it won't retain the same high performance for new games as when it was at it was a current gen card for new games that were release around same period or older titles, as Nvidia won't be spending the effort as they did then to optimised for new games- think of it as planned obsolescence.Well, some things to think about.
The 480 is faster and is improving all the time. For any future games been released it will be much better.
The 780 is going to keep going down in value, so if you sell it for a decent price now it might be the best you can do.
£199 is a fantastic price for the 480. It's a pretty cheap upgrade if you sell your 780. People have paid more for a less of a performance jump.
You will have a new 480 with warranty, which will be an easier sell further down the road if you decide to switch again.
Do it, a 480 at £199 is a great price, if you can sell your 780 for £70 or £80, then the 480 is only costing £130. It's no brainer to me.