RAM

I wouldn’t spend a lot on a RAM upgrade for that old of an iMac.

Also note that there is an Early/Mid 2009 that only supports 8GB and a Late 2009 iMac that supports up to 32GB.

If you do decide to upgrade, your best bet is purchase RAM modules from OWC.

https://eshop.macsales.com/shop/apple/memory/iMac

Are you trying to speed up the computer? In most 21.5-inch iMacs, slowness is due to the slow hard drives installed. Major "slow drive" clues are long boot times as and slow app launch times. If you have those symptoms, all the RAM in the world won't make very much difference.

I trust only Other World Computing or Crucial for reliable Mac RAM because they test in Macs. Their prices are as good as anyone else's too.

⚠️ Before you order RAM make certain of your iMac's sub-model. There are "Early" 2009 iMac and "Late" 2009 iMacs. They do NOT use the same RAM. You can find your sub-model from "About this Mac":

You're welcome.