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There are 2 ways you can go about this. But either way if you are going to do virtual reality its not going to be cheap. I would probably get new gear. But its a lot more expensive so do some research.
1) cheaping out - requires research, you might not be able to do this. I would do some reading. Because CPU's that fit in that motherboard can far outperform the minimum i5-4790k.
This avoids having to replace the motherboard. I probably wouldn't pick this path but its quite possible it will work its worth looking into.
You have a sandy bridge motherboard Find out if your motherboard can take Ivy Bridge CPU's. If you can update the BIOS to do that then look around for a I7-3770K. Yes the 1070 will probably work fine in your board provided its PCI-E 3.0 compatible. This is the cheapest upgrade of the CPU because you wont need to replace the motherboard, and ram and CPU. Just the CPU. You will also need to check if anyone has used an Ivy bridge cpu with an Oculus rift or HTC vive. Ivy bridge is an intel 3rd gen processor. If the minimum spec for a rift is a generation 4 i5 then I would check this out, it might have particular cisc instruction sets you need in which case you will need to go with option 2. If it runs find on a grunty 3rd gen I7 processor then I would just do that.
Run the Steam VR Test app: http://store.steampowered.com/app/323910
2) Upgrading the whole pc internals.
I would probably look at the i5-6500k. The K means its overclockable. If not get the CPU @networkn recomended. And it means you can go with the H170 instead of a Z170 motherboard to save a few $.
You will need to buy a new motherboard. And will probably want to go with one with a Z170 chipset. You can go with an H170 chipset but if you do this dont get a 'K' version of a cpu its just a waste and you wont be able to overclock. I would go with a Z170 motherboard if you buy a 'k' cpu.
You will need to replace your ram. Get 16GB you probably wont need more, although some people will disagree, get DDR4 its better, so check this on the lga1151 motherboards. Some still do DDR3.
Get a GTX1070 or GTX1080 the pascal ones in a few months after the supply issues are sorted out, and you can get a board partners one a bit cheaper than the founders edition. These are better for VR than a GTX 97x or GTX98x. As mentioned these wont be cheap. About $900~$1000 in NZ in a couple months for a 1070 and you might pick up a 1080 for about $1200~$1300.
The last thing you should check is that your current PSU can handle it. You might be fine here but I would check. And if its old or out of warranty I would look at replacing it.
http://outervision.com/power-supply-calculator If you need to buy a replacement PSU, I would probably steer clear of the cheap stuff get something gold rated or better. The EVGA SupaNOVA comes with a 10 year warranty. So its a good product.
Also remember the HTC Vive is $799 USD or an Oclus Rift is $599 USD, so in NZ dollars thats a lot more than a straight exchange rate conversion.
