korell: Okay, it is now confirmed, my GPU has had it. My techy friend came over this evening with his PC and when we put his GPU into mine it was all working fine, and even though we cleaned my computer and GPU up it still wouldn't work, so yes, the GPU is gone.
Now to decide whether to get a cheap replacement to tide me by or to just go straight for the full new PC.
Any opinions on the Skylake CPUs and LGA1511 boards? Is it worth going for the latest architecture or would it be more powerful going with a 4790K, say, even though it is older? Is DDR4 worth it? And which is better alongside a high end GPU?
On LGA1151 itself, very little to say. It's newer, better ( but not massively). The main interest is to be able to use DDR4. As far as I am concerned, DDR4 is not interesting right now. the additional perf you get from 2800, 3000 or 3200 DDR4 is not worth the price differential with 2133 DDR3, and won't for another year or two if the DDR3 story repeats.
On Skylake, from what I have seen, an I7 6700k is worth considering above an i7 4970K, but weirdly enough, if it's an I5 you're after, the 4670k remains a sweeter spot : the equivalent skylake offers a marginal perf increase, but is somewhat more expensive.
In the end, right now, I'd probably stick with LGA1150, DDR3 and the best 4xxx I could afford, with a better GPU and a good SSD as system drive, rather than go for skylake/ddr4/lga1151 if that meant compromising on the GPU and SSD. Because a higher end cpu needs to be paired to other higher end components to be effectively worth the expense.
now, all of this is based on continental prices, meaning a Z97 rather than an equivalent Z170 MB saves me 30-50€, ddr3 rather than ddr4 saves me 200€ on 32gb, and a previous gen cpu saves me 20-30€, which can allow me to have a 500GB ssd and a better GPU...