Hi guys, right now I have a 390x and I want to crossfire the thing, BUT! my motherboard only supports PCIe 2.0 x16 (x4 mode) in crossfire (is an ASUS Z97M-Plus) I know that, in theory I need at least both at 8x, but how much performance will be lost if I don't change my MB (I'm quite happy with it) Thanks!
This is a question that comes up a lot, and the answer is quite simple. Yes, you will lose performance - but will you notice? Probably not. You will lose around 5-6% performance using PCIe 2.0 x4, compared to PCIe 3.0 x16 - that's something that 90% of people won't notice. TechPowerUp have a great article that covers it in detail if you want to read it, but personally I think you won't notice the difference.
If you had a motherboard capable of x16/x16 in Crossfire, sure - you'd use it. But you don't, so to take in the full x16/x16 CF, you'd need to buy a new motherboard. I don't think that the cost is worth it, as it's only 5-10%. If the numbers were something like 20-30%, well my recommendation would be different.
So, if you don't change motherboard - you'll lose, at most - 10% performance. Considering you'll most likely gain 50%+ in upgrading to R9 390X Crossfire, I'd say the benefits are worth it!