So you can replace the harddrive? What if you want a better performing one? A larger drive? What if it dies? You'd be able to replace it yourself.
I'd love to see some gaming benchmarks on the 27". I have a hard time imaging the 4850 being able to drive games at the resolution with acceptable frame rates.
No you take it to the Apple Store and if you worry about that then you need to get AppleCare for the iMac.