Yes, it is. Again, you can't change the video card, because it is built into the motherboard. Opening the intel mac may invalidate the warranty, you should check about that.
It looks like it's a lot harder to open the new intel macs than to open the good 'ol 'blind' iMac G5s.
they won't - You can only upgrade the processor at the time of a custom order. There are no other ways to upgrade, and no after-market upgrades, yet. There is no optional video for the 17-inch iMac, only the 20 and 24-inch.
You can only:
upgrade to more memory (period)
You could replace the hard drive with more storage, or the CD drive with an up-graded model, if available - either of these would take you outside of warranty, and are a challenging job.
note - according to Apple service notes that I have seen, the Core 2 Duo in the newest iMacs do not have a socketed processor, it's soldered to the board. I don't know if that's true or not, but would take away that option, too.
I don't know if Apple will upgrade the hard drive (or other upgradable components), but I'm pretty sure that an AASP would - CompUSA is listed as one in my area, and they do upgrades..