I do lot of video editing work, mostly on FCP & Avid and I feel Quadro card is way to go. Do you think I can get it changed from Apple?
I don't even think the hardware would be compatible. Quite frankly, while you can open up the system and tinker with it, you have to worry about two things:
The iMac is a turnkey system, not an upgradable machine, like the Mac Pro (or, gasp, a PC). I'm sorry that this is not the answer you want to hear, but I don't think that Apple will do it. That said, it can't hurt to ask.
Some cards can be made to work in OS X with custom ROM, however, OS X extensions do not support the card's hardwired device ID, so it would result in a Kernel Panic at boot. If the card is close to another officially supported card in OS X or use the same GPU, there's an easy hack which will allow it to operate in OS X.
NVIDIA Quadro 4
This card is very close to GeForce 4 Ti series, using same GPU and PCB layout. Some even have dual DVI outputs.
Custom GeForce 4 Ti ROM will make the card work as a Mac Card, but a Kernel Panic will happen every time you try to boot with original NVIDIA extensions.
To begin you'll need to:
For the Quadro 4 700 XGL
Before physically installing your Mac flashed card, let's proceed with the Extensions hack.