Apple has released a new document about the upgradability of the new iMac G5:
Think you need a new part? You can replace many of your iMac G5's parts yourself.
The iMac G5 is designed to make it easy for you to install replacement parts if you need to. The parts you can install yourself are:
AirPort Extreme Card
Memory - DDR 400 MHz (PC3200) SDRAM
Mid-plane assembly (contains the main logic board, the G5 processor, fans, NVIDIA graphics processor, and so forth).
This is very interesting and good news. Essentially, the entire new iMac G5 is user replaceable. The new iMac's design is a far cry from that of the previous iMacs, which were essentially black box designs with very limited user upgradability.
This potentially addresses the issue of the iMac G5's less than stellar GPU. For some users, the GPU is the unit's Achilles heel and the various Mac enthusiast sites are full of comments lamenting Apple's decision not to offer higher speed GPU options. The possibility of a future GPU upgrade can help future-proof the machine (as would a CPU upgrade). However, it sounds like a GPU (or CPU) upgrade may require the replacement of the entire mid-plane assembly in the iMac G5. If true then one wonders if these components will be widely available, and even if available, how expensive they would be.