I just bought a 17" Core Duo iMac a couple weeks ago - no regrets, the update is nice, but I bought from Amazon (for the $100 rebate) and the upgrade to the 17" just isn't that dramatic . If I'd been in the market for the 24", that would be a different question, but that won't even fit on my desk, so I'm using the 17" spanning onto my 21" flat panel.
So, the new iMacs all max out at 3GB of RAM, where the Core Duo machines claimed 2GB. Since the Merom is pin-compatible with Yonah, it seems to me that the Yonah iMac may actually be able to take 3GB as well, but Apple didn't mention it because prices for the 2GB SO-DIMM were astronomical at the time (they're still very high now, of course). Since the insides of the new iMac don't seem to be changed from the Yonah version, is there any reason to think that there was a change that would effect the maximum RAM?
My iMac is already maxed at 2GB, and I'm only really curious about the future ability to upgrade - at current prices, it wouldn't be worth it, but in 8-12 months, that could change.