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This is a fairly standard Mini-ITX Intel/Nvidia Gaming PC, so i'll skip the obvious stuff and go straight to the details.
The Corsair 250D was originally designed to fit Corsairs original H100i, which uses a 27mm radiator. The H100i v2 and H100i GTX use a 30mm radiator, and will not clear the motherboard. I originally fixed this with my v2 by cutting the corner off one of the fans to clear the cpu power socket, but the fan still hit hard against the vrm heatsink. The old v2 was starting to get noisy anyway, so i switched it for the h100i pro, which returns to the 27mm radiator. Now the fans will barely touch the motherboard tray when installing the radiator, but will not hit the motherboard itself.
The tubing of the AIO will hit the 140mm front case fan if the tubes are routed to the front. Tubing will also hit the heatsinks if you route them to the rear, and put the fans outside the radiator. So fans inside the radiator as exhaust is the only practical setup. The tubes also block one of the 80mm rear fan mounts. A 200mm front fan will fit with a shorter graphics card, but not with an oversize strix card.
I waited until the RTX 20-series cards came out to upgrade my Asus 1070 Dual from my previous build, but the majority of those cards are 2.7 slot coolers, which will absolutely not fit. I bought an Asus 1080 turbo (blower-style) that dropped right into the slot, but was very noisy. The blower fan clicked at any speed, and at full blast was unbearable, so I returned it and swapped to a 1080 Strix.
PCPartPicker lists the 250d as having 290mm of graphics card clearance, which is partially true. The opening of the top of the frame is 290mm, but the actual frame where the card sits is 300mm. The Strix cooler is 298mm, and will fit barely, but will not drop in. I ended up having to remove the motherboard and front fan completely, dropping the card in at an angle and holding it in place, while sliding the motherboard back under the card. The motherboard screw under the card was the hardest part, and required a long, thin #1 phillips, starting the screw gently at an angle while holding the Graphics card up for clearance. Some people have had luck with just removing the cooler and ram. There are several millimeters of clearance around everything once it is in place, and I had no heat issues outside of Furmark. The AIO will exhaust all the 1080's heat at the expense of water temps if pushed too hard, but for games and 3dMark level tests, its fine.
When upgrading from the 1080 Turbo to the Strix, I needed another 6-pin pci-e power connector. But alas, I have lost the extra cables from my old power supply. After buying an incorrect cable for a corsair supply that wouldn't fit, I finally got mad and bought a full Cablemod kit.
That is the correct kit for my Seasonic SS-660XP2 platinum 660w power supply. Cablemod Lists it as a P-series in their compatibility list.