That's also not a "killer" desktop. The 290 is a a 2.5 year old card. The GTX 970 is equivalent to a 680 or 780. The 680 came out almost four years ago. If you had no computer at all, you could build that system for ~$700 - $750 ($200 CPU, $100 Mobo, $200 Used GPU, $50 RAM, $75 SSD, $100 Tower+PSU).
But that's immaterial, as most early buyers of the Rift have hardware that meets the requirements above. And four USB ports? A $10-$20 powered USB hub solves that. Wouldn't surprise me if it came with one.