Yes, the parts list I put together includes both a case, motherboard, and power supply capable of supporting two GTX 980 cards in SLI. I did not suggest an R9 290X at the time because it seemed that you wanted high framerate at high graphics settings, a PC built around potential twin GTX 980 cards would seem to be better capable of doing that than a system built around potential crossfired R9 290X cards would.
Comparing my build to the one you linked, they have the same processor, but the motherboard, RAM, storage drives and graphics card are somewhat cheaper or inferior. If you were interested in saving some money, you
could go with twin R9 290X graphics cards which would cost much less than twin GTX 980 cards yet not be that far behind in performance for
some games, although not all. However, you mentioned Project Cars and Assetto Corsa to be very important to you.
This benchmark from May 2014 seems to say that Project Cars does not work very well with crossfire (AMD multi-GPU), it ran faster with SLI (Nvidia multi-GPU).
German website pcgameshardware.de released the results of their testing from January of this year and the R9 290X appears to perform worse than the GTX 970, while the R9 295X2 (two R9 290X cards in one) does not perform any better than the single R9 290X. Fairly disappointing, looks like crossfire isn't worth it. Looking toward Assetto Corsa,
pcgameshardware.de also made available their benchmarking results and the R9 290X does take the top crown, with the GTX 980 being left in second place.
In short, if you were planning to build a PC for those two games in mind and had a choice between a GTX 980 or an R9 290X with the possibility of picking up a second identical card in the future, I'd recommend the GTX 980. If you were to run twin GTX 980 in the future, at least that works for both games. If you went with the R9 290X, it'd be pointless to pick up another one in the future for crossfire because it wouldn't work very well for Project Cars.
Edit: Thanks Liljoka and lordfuzzybutt, we've covered all sides of the comparison. I was hoping someone would do the hardware comparison while I dredged up graphics card comparison articles for racing sims.