Using Win 8, my 7900GS shows artifacts with any game using SM3, all other games that don't use SM3 work just fine. Games doing this. 
 
Company of Heroes
FarCry 1.4
FarCry 2
GTA IV
 
are so far the game's that have done this, the rest are either to new for a 7900GS to run anyway or are older. Any idea what could be causing this, or is my old card just at the end of its life.
 
also I'm aware this is Geforce 8/9/200 but we don't have a geforce 7 section
 

You could always download and run EVGA's OC Scanner software and use it to scan for artifacts. If it finds some then you should try a clean install of drivers for your card, but if it still finds artifacts after that then the card has probably reached the end of its life. 
 
If the card is not under warranty and you're really desperate then you can try raising fan speeds or boosting voltages a little to see if either allows the card to pass the OC Scanner artifact check. 

I did test it again back on the Athlon 64 with Windows XP drivers, 90 series, and no problem.