So I realise something, my fast render pc has only 16GB's ram.
BUT, my old modelling computer has 32GB's of ram.
And both my computers use DDR3 Ram.
So I had to open up both computers and swap them.
Long story short, my faster computer can now render with its graphics card, without running out of memory!
And I can still take single shot test renders with my old pc, using the cpu.