Quick hypothetical question...

[driftrunner007]

Let's say that I had a spare GPU with 4gb of GDDR5 memory and I was willing and able to do anything including messing with the logic board to get the 4gb of vram to work as regular ram. Could it be possible?

 

[Gam3r01]

No.
They arent even the same kind of memory (GDDR vs DDR), and there would be no way to get the system to address the GPU memory as normal RAM.

 

[Hellfire13]

GDDR works at a much higher bandwidth than DDR. GDDR is optimized for GPU - multiple instructions per cycle, whereas DDR is optimized for CPU - single instruction per cycle.
AFAIK, GDDR cannot replace DDR in your machine.