I am looking at a replacement pci-e card for a m2n-sli motherboard. In the description for it, a gigabyte gv-r 687 OC-1GD Radeon card HD 6870, it refers to pci-e 2.1 . Will it work on my slightly old mother board?
It refers to the version of PCI-e x16 it has. It's the transfer speed and systems involved in the slot.
1.0 has a transfer rate of 2.5GT/s, 250mbps data rate.
2.0 has a transfer rate of 5.0GT/S, 500mbps data rate
2.1 has the same speed as 2.0, with technologies being incorporated into 3.0
3.0 has a transfer rate of 8.0GT/S, and one of the technologies is removing the 20% overhead that was on 2.0, so it effectively doubles the transfer rate of 2.0.