iMac G5 comes in 17" and 20". No 15" there 🙂 and the video ram is 64 🙁
Although the only problem i see with the video card being upgraded is the Powermac. The Powermac's lowest Video ram is 64 MB. iMacs are meant to be for casual users, not for hardcore video stuff or graphics. My take is that they'll upgrade the powermac before the iMac G5's videocard.
If you are worried about the video card you could probably buy the low end Powermac (Single 1.8 G5) and a cheap moniter. the video card could be upgraded to match your likings. and as time progresses you could add ram and maybe buy a Apple moniter 😉
Course we have no idea really. Not until the words "New iMac G5" are spread across the front of apples web site than will we know for sure 😱