I have to build a 5GbE link with a SFP+. Can i use the one that is rated for 8Gbps? And what kind of NIC should I use on host PC?
Both 5 Gbps and 8 Gbps are non-standard ethernet speeds, and you won't find those speeds on SFP+s. In fact, you will not find 8 Gbps ethernet at all. If you are using fiber, you will get 10 Gbps, but you must make sure of the fiber and length, which must match the SPF+, which must also match the SFP+ on the other end. Some copper SFP+s can also do 1 Gbps (possibly 100 Mbps and 10 Mbps), besides 10 Gbps.
Some switches from members of the NBASE-T Alliance have ports which can do 2.5 or 5 Gbps, besides the standard 10 or 100 Mbps, and 1 or 10 Gbps.
It is unlikely that you will find a host NIC that can do either 5 or 8 Gbps. There are server NICs which can do 10 Gbps.