look for a cheap 256 stick from a Mm owner that is upgrading...
Then when prices on 1GB dimms drop, buy a pair.
This way you have paired ram for the iMac and the Ram Bus will run at double the bitrate...
(that is what I am doing anyway)
also, you can sell the 256 pair to a G5 owner for the price of the original 256 upgrade (or more, who knows) so you won't really lose anything. (assuming you can find a buyer)