Many VR modes in games are stupidly, stupidly demanding. Like, to play a game at 1080p/75fps, you're really having to power the game at 1500p with a peak of 90fps(to keep it so it never drops below 75).
I've pretty much decided to wait until CV1 is out (what, 12-15 months from now?) before building my gaming PC. I'll get a lot more bang for my buck by then and it'll be clear whether CV1 is actually everything it's promised to be. Project Cars will be a year old by then too so there should be some expansions out and bug fixes/updates firmly in place.
My 360/PS3 backlog and laptop racing sims should keep me entertained until then, but the wait will still be pretty torturous.