Q: Upgrading older IMac

Hi,

El Capitan requires a 2007 iMac or newer >  Upgrade to OS X El Capitan - Apple Support

I'm suprised you running on 1GB RAM at all.

Normally i'd suggest updating to the latest system that supports your specs That would be 10.7.5, but I think that 10.6.8 is the more stable system and with the lack of RAM you should stick with it.

If you are indeed running a White Late 2006 iMac, you can upgrade RAM up to 4GB and replace the HD for a faster SSD.

https://eshop.macsales.com/shop/apple/memory/iMac/Intel_Core_2_Duo_2006

https://eshop.macsales.com/shop/ssd/owc/imac/late-2006

However, your Mac will only be able to update to Lion (10.7.x).  But unless you already have a download code for Lion, there is no way to buy one now from Apple.  So basically you are stuck at Snow Leopard, but you can add more RAM to it.

Otherwise, after 10 years, I think its a good time to think about updating to a newer model.