Nice computer. Hard to say on the ram. 8 is probably more then enough unless your doing video editing or gaming. Maybe even enough then.
For macs I have only ever bought laptops. All three of them I was able to upgrade the ram myself. Not sure if I need more ram or not, but it was a lot cheaper for me to upgrade the ram myself then at the time I purchased the Macbooks, so I did. Besides that the Mac store said the max ram I could upgrade to was half of what the internet was telling me. For example I by a macbook with 2GB or ram, can upgrade it at time of purchase to a max of 8GB. Research online and at ram store outlets tells me I can max it out to 16GB. Twice the ram for about half the price to do it myself.
I know you said on your new mac that's not a option. I see a lot of computers going that route for some reason. I purchased a Acer laptop last summer on sale at Target, dirt cheap. It only had 4GB of ram in it. I knew I would need more for some programs I run. Thought I would just upgrade it myself as I have always done. Come to find out it's not so easy to do. It can be done, but you have to rip half the computer apart as it's not really made for the consumer to replace or upgrade the ram.
All depends on what your going to use it for but if I was buying a Mac that I could not upgrade the ram myself I would probably max it out at the time of purchase.