AC ran on my old GTX 560 on med settings, so I don't think the rig has to be that good
For rF2 it probably does though. Think about how well, or poorly, rF2 runs on your machine vs the same machine running AC. For me, running rF2 is significantly more demanding on my machine to get pretty graphics and steady FPS vs what can be achieved in AC. The DK2 requires 75 FPS to achieve the stutter free buttery smooth graphics seen in many youtube videos. This means several things in rF2.
1. You have a beefy machine that can run max settings and get 75 FPS on two screens (one render per eye)
2. You knock down the settings to get the required frames and have less eyecandy.
3. Your machine cant handle the rift at its current FPS requirements and you start puking all over the place due to high latency and poor frame rates.
I suspect that most current players would fall into category 2 or 3. Those running rF1 likely are firmly planted in category 3 but that's a different thread all together.
When a person can run a game like AC with the requisite FPS vs the same machine not being able to on rF2, it is telling about the engine running the sim. Personally, I think that a lot of the rF2 resources are used in the physics side of things and that the graphics engine is secondary to that goal. This is why no one doubts or even tries to argue that rF2 is anything but the king of physics in Sim racing today. That comes at a cost though. Graphics. The DK2 needs a specific FPS as listed above. I don't thin that ISI has optimized the graphics engine to handle the needed FPS on lower/Mid-grade machines. Mostly because I dont think they think graphics matter. With the DK2 they do. This also may be the reason that they are somewhat quiet on the matter, that and they have the commercial client to keep happy.
As an aside I am building a new machine with the Rift CV1 in mind. (8 core AMD FX CPU, 16 GB ram, R9 290 4GB Video Card) I plan on running all my games, if possible, through it when it comes out. This includes rF2 if I am still playing it when the CV1 comes to market. I hope so. However, like some other racing games I have played in the recent past (SRW, ACR) rF2 may go into the "has been" bin if it does not officially support the Rift. Too bad too since I really do enjoy the game.