I did not know that, thanks.
My current iMac is a 2010 model i3 processor (1.6 GHz I think) with 4MB ram and a 500 GB HD that's pretty much at the straining point. It's making a lot of noise, which it hasn't done in the past.
RedCroissant, your points are valid. For some reason I seem to recall that I could not expand the memory on this model. Need to check on it.
It's probably making a lot of voice because either the HDD is full, or you upgraded to Yosemite/El Capitan. Either of those negatively impacts the system (the HDD being full for the lack of virtual memory). Upgrading to Yosemite is only a negative if you don't have enough RAM or enough free space on your HDD (which in your case is both).
The current model is the 1.6GHz dual core i5 and you are correct that you can't upgrade the memory after the fact. However, THAT iMac is essentially the same machine as a MacBook Air and it does not give you a drastic performance boost that would justify spending $1200 on a new Mac with comparable performance specs, inability to upgrade, and with a display with the same resolution as yours.
Now the retina 4K 21.5" is a much more powerful machine and definitely an upgrade. The fact that you want to configure it with a 1TB fusion drive though makes it cost $1600.
You, however have the Mid 2010 3.06 GHz i3 and it is absolutely upgradeable. You have a potential max of 16GB of RAM that is installed underneath in the middle of the device that you can easily access with a regular phillips screw driver. The Hard drive can also be upgraded to a drive that's at least double the storage or if you don't use the superdrive, you could even put 2 drives in there (one for the boot volume that could be a SSD and installed where the superdrive currently is, and the other one connected to the other SATA connection).
For ~$400 you could have
$120 16GB RAM
$115 240GB SSD
$80 2TB HDD
$30 Data Doubler (for installing a SSD in the superdrive bay)
$35 External Optical Drive
I know that $400 sounds like a lot, but even that is still 1/3 the cost of the base model 21.5" 1.6GHz model and 1/4 the cost of the retina 4K model!
If I were you, I would seriously consider upgrading your current machine. And if you happen to live in Orange County, CA I would even upgrade it for you once you get all of the parts.