After actually looking at the xPlane 10 system requirements, the dev says
xPlane10 will use just 1 core.
THAT ^ is THE key deciding factor.
That means an i3 would run it just fine. Intels have better single-core performance.
However... <- you knew this was coming if you have any sense XD
Even in 1-2 core games like Skyrim, @ 1080p the i5 is only marginally better than the FX series CPUs. And the FX series CPUs can clock a bit higher than the i5 and i7 on equivalent cooling, making up a lot of the difference. And as much as I love Tom's, I don't just get my info from one source. This is based on current testing from multiple sources.
Any time you're looking at spending a significant amount of money it's good to weigh the pros and cons of all your options. One of them should involve your future plans for the investment. More and more devs are going multicore. Even the xPlane dev has a post on their site about multicore questions. And if you're building a new PC, the current performance levels of the FX series CPUs with windows driver and patch updates is on par with an i5 and i7.
But in the end, if he wants to spend his money on a 6 core Intel extreme edition CPU to go with his Titan, that's his call. We're just giving him all the info he needs to make the decision.
PS - Agree 100% with Sangeet's statement above, which is very close to the point I'm trying to make here.