I use a 20" iMac at my church office (and another PC) and just purchased a 24" iMac for my photography business. The unit I purchased (not from Apple) was configured with the 7300 GT, 128MB graphics card. Lots of rumors out ther (many of which I've explored, looking for more than speculation) about whether one can upgrade to the 7600 GT, 256MB card. Of course, you can configure a new purchase from Apple with this upgrade, but can it be done to existing 24" iMacs once they ship...or if you purchase one with the 7300 card refurbished, used, or new from another party.
The answer is "YES". This comes first hand from Apple technical support department (which I spoke with today). Here is what they told me:
1. This upgrade can be done after purchase
2. It will not be performed by any Apple store (they just won't do it). As to why, your guess is probably accurate.
3. It can be performed by an authorized Apple reseller/service center. They can (whether they will may be another issue) order this part (the 7600 GT card) and install it for you. The Apple Technical Support representative assured me that there is a part number that is availalbe to them if they chose to order it and perform this upgrade.
4. What do you do if your local authorized Apple reseller/service center refuses? Apple Tech Support gave me this advice: keep inquiring with other Apple resellers until you find one that will let you give them your business. According to Apple, they are out there.
So, there you have a little "Apple Insider" news. Gentlemen (and Ladies), start your engines!