I posted this on macrumors as well. I'm considering upgrading my older IMac, maybe a SSD and upgrade ram. I would use it for work, MS office work, mainly heavy excel, email, Remote Desktop. Also for photo editing, a mix of Lightroom and affinity photo. With the stats below worth upgrading?
CPU: 3.06 core duo
RAM: 4GB
Graphics: NVIDIA GeForece9400 256 MB
My other computer is a work computer that I can use its a MBA 4Gigs of ram 1.6 I think processor.
The Core Duo processor is pretty old ... I'm thinking 2007ish?? As I recall, it's a 32-bit processor, so it's not able to recognize more than the 4GB of RAM currently installed. I still have my first MacBook pro ... with a Core 2 Duo processor, and it only recognizes 3GB of the 4GB of RAM installed.
So that said ... yes, Lightroom would LOVE for you to have more memory installed. I would suggest 8GB as a minimum, and as Marcel said, 12GB or more would be ideal. I run 16GB of RAM on my laptops, and 32GB of RAM on my desktop ... but I don't notice a significant speed improvement with Lightroom on the desktop, so 12-16GB is probably the sweet spot. Lightroom also can take advantage of the GPU on your graphics card, so an iMac refresh would help there.
I don't think you'll see much improvement for your email and remote desktop use, but photo editing will for sure. Depending on the functions you're using in Excel, you may see a performance improvement there too.