Will PCIe 3.0 matter?

So I managed to get an unopened, in-box 8350 from a friend for free. I plan to get a 290 and do a build with it that should last me for a few  years.

The  only issue is that AM3+ boards with PCIe 3.0 don't exist. Am I going to be screwed for the next 4-5 years when I do GPU upgrades if I don't have PCIe 3.0?

Highly doubt it, what will matter in the future is new standards like Sata-express which utilize PCIE lanes

290 will be fine on PCIe 2.0.

I think you would need 3.0 for dual 290s

For one thats not true. Asus Sabertooth Gen3 R2 has PCIe 3 For Amd boards. Secondly, it does help a small amount now( Not alot ) and will mean more in the future. Thirdly, You are fine for now with that match up.

SATA 3.2 and DDR4 that's where it matters, technically you can run dual GPU's on a PCI-E 3.0 lane and still have some headroom.

Soon you're going to see a unified bus access. Currently there is the CPU in the middle of the North Bridge and South Bridge.  I know Intel doesn't call it that way but they still work that way.  AMD is coming out with HSA using hUMA and that's a step towards that direction.  PCI-e is theoretically faster than RAM and has a larger bandwidth so i could see RAM being powered by PCI lanes and same as SATA.

Piledriver has PCI-e 3.0 string and the irony is that ASUS is the only motherboard manufacturer that made a revision to AM3+ boards with the Sabertooth Gen3 R2.  I think that's why there isn't a steamroller CPU yet, they're waiting for AM4 or possibly FM2+/FM3.  Not sure yet.

Soon we'll see that, possibly next gen AMD or Broadwell possibly.  PCI-e 3.0 is quite useless for graphics cards atm because, PCI-e 2.0 is quite sufficient even for a 7990 or 290x with no problem, just make sure you have both lanes @ x16.

The only issue is that the Gen 3 R2 is literally impossible to find ;-;, it was a limited edition run

Ok, great advice. Because I only plan to use high-end single cards, I'm going to be fine with PCIe 2.0  x16.