The regular home PC, is built out of standardised parts and has standardised expansion slots, and form factor (ATX) and parts can be for the most part easily interchanged. (my power supply went bad, and it took about 8min to replace it).
Imacs don't have a PCIe slot, what ever they have is different works as well i guess but not a standard PC type PCI-e slot.
I do not think your model and year was offered with an nvidia card option just ATI.
I assume by service you mean apple offers an upgrade to the ATI DHD5750?
Yes you could have it upgraded to a better ATI card that is designed to work with your model imac.
This forum would explain it better then i could:
As you can see, from the forum this would not be something easily done if not at all, it is difficult to work on/upgrade replace a video card in an imac and if a mistake is made it could be extremely costly.
Also not sure the upgraded ATI card would be any better for gaming. I don't really do much gaming anymore and I don't use ATI, You would have to look to see how much better the other ATI card would be. It might be a lot better.
Mac parts are designed to work with mac os and their own proprietary hardware design (physical design) and software design.....this is good. With Apple keeping strict control of what products are offered from the factory in their models, and what is supported, keeping that nice Mac OSX running smooth is a great deal easier and it works, mac OS works. They could make it just a bit easier thou, that's just my opinion.
There is a difference between nvidia and ATI, they have been battling it out for quite some time, and some prefer ATI and some Nvidia.
You could try and reduce the native resolution you are running before playing a game, and if your running the game at 2600 x something, change it to 1920 x 1080 or less, play with the shader options etc....see what it can really do if you haven't done that already. WOW shouldn't be all that demanding on the computer unless i am not up to date.
pushing all the options in a game, a new game like the new call of duty i guess, to high, at really high res. not many systems without SLI setup or a fancy computer with a $200-300 card and 600watt power supply could do that with good results.