Do you plan to re-use the case or move on?
If I'm going to build a completely new system, I'll also want a new case. I think the old system is still very salvageable, so it's going to need a case anyway. If I'm going to give it to charity, or maybe to a friend with more time than money, then I'm sure they'll appreciate this nice case. It is going to need a bit of cleaning and dusting, because I'm afraid I neglected that over the years.
What's your monitor resolution? Do you have more than one monitor?
Just one, 1600x1200. I'm open to the idea of getting a bigger or otherwise nicer one in the future (maybe one that's easier to clean). I don't need a second monitor on this PC, though it's always nice to have the option available.
Do you want to max out all the game eye candy
Some of my favourite games are 2D or very retro. Heavy 3D games like The Witcher 2 look fantastic at the very lowest settings. The heaviest 3D games I have are The Witcher 2, Medieval Total War 2, and Civilization 5. (Of these, Civ 5 was the only one that gave my old system problems, but turning down the settings to the minimum solved that.) I do like pretty stuff, but it seems 5 year old technology is easily good enough to support all the pretty stuff I want.
So that's no on maxing out all the eye candy, I guess.
is there a budget (or wattage!) constraint?
There are no hard constraints anywhere, but the lower wattage, noise, budget (and space!), the better. I think the most important things I don't mind paying extra for concern comfort: low noise, reliability and low maintenance.
Do you want to overclock your CPU/have the ability to overclock your CPU?
No. I've been toying with the idea when I got my old system, but I don't think I ever actually overclocked anything.
Ideally, I'd like to order the parts including assembly, and have the shop that assembles it not be able to mess anything up about the assembly. So the more standard, the better. (I did that with my previous system, and it turned out fine. Actually I think I did send it back once when it was just new. But after that it was fine. Not having to send it back at would be better.)
Witcher 2 likes quad cores and a hefty GPU. I'd expect Witcher 3 to be similar.
The Witcher 2 runs fine on my old dual core and 5 year old GPU (except for the occasional blinking bush), but I expect the sequel is going to be pushing it a bit too far.
The last couple of generations of gfx cards have decent idle power - typically 10W.
That's good news. I think I once saw a graph that showed idle power consumption of the HD3850's successors to steadily climb every generation, but a bit more than very low is still going to be pretty low, I guess.
If you plan to game at 1080p, the GTX 760
is the card to get. Specifically, the MSI Gamer version seems to be the best OEM cooled card.
At 1600x1200, I've got a slightly different shape, but similar resolution. But what if I go for a much higher resolution? Or is 1080p really the standard you should stick to for gaming nowadays? It seems a bit odd that resolution doesn't really go up at all; I'm used to 1600x1200 for 2 decades now. Shouldn't we be getting super high density "retina" displays now?
I'll definitely look into the GTX 760.
Edit: having looked into the GTX 760, it's quite a bit more expensive than my HD3850 was. I paid € 120 for it (normal price was probably around € 150 or thereabouts), whereas the GTX 760 seems to cost around € 240. That's a significant difference. Do I really need something that expensive?