You can add RAM yourself, and i'm guessing you can exchange the hard drive as well.
But it's an Serial-ATA connector, so you can't just stick an old one you have lying around in there.
I'd buy it built-to-order from apple, increase the hdd size and then later add ram yourself (ram's usually cheaper from non-apple retailers)
If it were me...I would just get an AP and BT as BTO options and leave the rest stock. You can get an upgraded HD and RAM at a later date in time for MUCH cheaper. As far as the GPU goes...maybe apple will have a replacement GPU program..where you send them the Imac and they replace it for a fee.
The graphics card is part of the motherboard, so it's not replaceable.
And no, Apple would not replace the GPU, as GPUs usually are not compatible between each other. Anyway, I never heard of any company replacing GPUs (Graphic Processing Unit, it's only the Graphic processor, without the rest of the video card)...
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