Although I have not scientific data to support my theory, I always go with 2 generations. Usually within 2 generations a new connector will come out for the video card, and to replace the video card would mean replacing the motherboard, cpu, and processor. For example, your DP45SG motherboard has PCIE 2.0, and the current is PCIE 4.0, which is 2 generations ahead. Normally to upgrade and get the full effect of a new video card, you would need to upgrade your current processor, motherboard, and RAM. However, you are lucky as all new PCIE video cards are backward compatible and could work on your board, but they would take a reduced performance hit because of the limited bandwidth of the older PCIE slot.
Another thing to look for is maxing out your cpu processor while playing a game or using an application. That's a good indication if you need to upgrade that.
Thanks dude ..