The info I have suggests that PCIe x8 (40Gbps) bus bandwidth can get saturated quickly when running Resolve 8.x. If no Gen2 x16 slot available, I suggest going with new system. Based on specs for several 3rd party workstations I’ve seen so far, you could still use an Xpander for adding multi-GPU performance to Resolve 8.
Eric Fiegehen
Cubix