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To answer that question, you need to know just how busy your servers are going to be and what their traffic characteristics will be.
One "easy" answer is to compute capacity assuming minimum size Ethernet packets, and all ports running at 100%. For 8 Gbps (duplex) you would want a switch that has a non-blocking fabric that provides 16 Gbps (twice the port bandwidths) backplane capacity and 11.9 Mpps (1.488 Mpps per Gbps).
Real world (especially edge) switches rarely run this kind of load, but if the switch supports it, you're guaranteed wire-speed or line-rate. (Many "modern" Enterprise grade switches support wire-speed/line-rate.)
As I mentioned in my initial post, besides bandwidth and PPS, there's buffering resources to consider. In real world networks more than one device may transmit to the same device or one device might have a higher bandwidth connection. Then it's not a just a question of raw performance, but how much of a data the switch might be able to buffer.
Just "knowing" 8 servers with gig NICs isn't enough information to select the "best fit" switch.
