GPU is HOT, planning to open a hole and mount a Fan

Already opened it, cleaned it (not too much dust really, nothing clogged), all the fans work great im using SMC Fan Control, and with everything blowing at max rate the gpu is still idle at 50-60 C, and with work it goes up to 78-80 C, i removed the heatsink and replaced the thermal paste, before that temperature will go up to 90+ and the gpu will halt with graphic glitches first.

So im considering the possibility the heatsink pipes could be the problem and blowing air directly to the gpu could be a cheap direct solution..

Sounds like a bad GPU.; If you don't mind loosing your resale value you could give it a try. But you will disrupt the normal air cooling flow. Better to try something that would suck rather than blow. If you could suck air out of the rear vent that would probably work better.

Another idea is removing the CD player, which i don't use anymore and don't know if its still working really, and create with tinfoil some kind of duct do direct the airflow from that cooler directly to the gpu, .....

Did you clean and apply a fresh coat of thermal paste on both the CPU & GPU's? If your GPU is really cooking here I suspect you are driving the system to hard given its design. Remember, when this system came out heavy graphics used in games was just starting out, this system was not designed to support that heavy a processing load. Sorry

If you are a gamer I would strongly recommend you look at getting a newer system that can support what you are running better. Trying to redesign the systems cooling isn't that straight forward as drilling or ducting air flows.