I'm slowly updating my computer from very very old to just very old 
I used to have a AMD Opteron 2Ghz and a 8800 gts.
I have picked up a few bits second hand and now still have the 8800 gts but now with a core duo 2 E5500 2.8Ghz, with a Gigabyte G31 based motherboard, 4Gb of ram and recently added a ssd Hard drive, it's certainly a lot faster and have enjoyed tinkering with it but the gfx card is now showing it's age.
So the next upgrade will be a second hand gfx card but is there any issues with the motherboard and newer gfx cards? I will only be looking at the cheaper end of the market so 660gtx or similar but if say a 970gtx would work even at a slower speed as the PCIe slot will be an issue I would save the extra money a get a faster card and upgrade the motherboard and cpu at a later date.

I used to have a AMD Opteron 2Ghz and a 8800 gts.
I have picked up a few bits second hand and now still have the 8800 gts but now with a core duo 2 E5500 2.8Ghz, with a Gigabyte G31 based motherboard, 4Gb of ram and recently added a ssd Hard drive, it's certainly a lot faster and have enjoyed tinkering with it but the gfx card is now showing it's age.
So the next upgrade will be a second hand gfx card but is there any issues with the motherboard and newer gfx cards? I will only be looking at the cheaper end of the market so 660gtx or similar but if say a 970gtx would work even at a slower speed as the PCIe slot will be an issue I would save the extra money a get a faster card and upgrade the motherboard and cpu at a later date.