Depends...
Tell me, can I play NHL 2002 on an iMac? How 'bout Medal of Honor? Links 2001? Command & Conquer?
Don't know 'bout NHL; personally prefer to DO sports; never have gotten into PC (or Video Game) versions of football, baseball, what have you. But that's neither here nor there. To continue: never heard of Medal of Honor, Links = yes, C&C = yes. Whoops, my mistake. I could actually handle any of 'em with Virtual PC. So, yes is the answer.
Can I modify the image quality of the video card like this? Can I change the video card?
You can do anything anyone can do with an NVIDIA GeForce2 MX GPU. Prob'ly can't change the card on the iMac, though you can on the G4. You can swap the AGP card or add additional video cards in the remaining slots (although they would be limited to the PCI bus)
Can I modify the case, like this or like these?
If you have a can of spray paint or an airbrush.
Can I tweak the bios to improve performance? Maybe even overclock the processor to gain more speed?
Yes, though that would void the warranty.
Without degradation? With complete compatibility in full resolution?
Yes, though that would void the warranty.
As I would with any PC mod I make...