i just wondered what file/files would i copy and paste to do this? would it just be the installation files that are downloaded plus any .bin files they many need?

You need to put on the disk all files you have downloaded. It include all exe, bin and zip files (in case of extras).

You have to copy installer and proper .bin files. Most of the installers don't need any .bin files though. That's it.

For the DOSBox versions of games, I think you can make a 'playable' backup by copying the folder where the game is installed, along with the shortcut that launches the game. Just gotta change the properties on the shortcut to reflect the location of / on the backup.

You mean that many installers are self contained (if under 2gb) and dont HAVE .BIN files rather than just not need them You only need the zips if you're backing up extras too. Sensible thing to do but not essential to the game itself

IIRC, if you copy straight off a CD then the files will be read-only so make sure to take the read-only attribute off when copying from the CD.

Yes, I said that. :)

A lot of GOGs will fit on a single 4.7 GB DVD-R+/-, so you don't waste too much space separate a lot of your small GOGs into directories of roughly that total filesize and burn away. May I also suggest a Synology NAS (stands for Network Attached Storage) for quickly accessing files you don't have on your main machine. They aren't that cheap but you can get a small one set up for 200-300 dollars and it's actually a very good investment for anyone who has multiple machines at home (beats futzing with discs too).

You should still do disc backup (maybe even offsite) even if you get a NAS.

I normally download all the files to each individual game folder. Then copy to portable hard drive and physical media.

As mentioned in the thread above - the zip files are for extras - manuals, soundtracks etc

currently i have a folder dedicated to bonus content for each game but let the downloads go elsewhere

As an alternative to a NAS, a USB hard drive also does the trick. I have my installers and add-ons backed up to the USB drive, and installed the older and less drive-intensive games on that drive, as well. Works great for the DOSBox games that you can run anywhere, as would installing them to a thumb drive. The NAS is nice if you want to access a bunch of other stuff on multiple PCs, though, something a USB drive isn't particularly good at.