Thanks for that, I managed to find that RetroPi image not long ago so I'll install that tomorrow. I want it to be as close to a console experience as possible so I'd much rather it booted straight into it..I would just install the retropie image. Here is the RetroPie site. It won't run a full desktop environment, so it saves some space and overhead. If you actually want a desktop environment though, Pixel is a fine choice.
I haven't needed to do any overclocking for the stuff I'm playing, but you may as well if you have heatsinks. Not sure what a safe overclock is without active cooling, but hopefully someone else can chime in.
I'm assuming that because everything is booted from the SD card that I'd be able to have a second SD card with a full OS installed so that I could choose which to boot into? Obviously, after powering down. I do want to learn more about the Pi so I think that it would be handy to be able to do that.
People do seem to overclock their systems but I'm not really sure of the real world benefits in RetroPi, unless somebody on here gives me a good reason not to I'm very tempted to get one of the cases with a fan built in. This was just after the Pi 3 launched but this feller has managed to overclock the CPU from 1.2GHz to 1.45GHz and the SDRAM from 400MHz to 500MHz only using heatsinks like I'll have. What I am reading though is that it can be really variable from Pi to Pi, so what is stable on one isn't necessarily stable on all of them.
I'm not wanting to do anything risky and it all comes to how much of an advantage I'd get in a real wired situation.