@Charlottesmom Error 12 has little to do with how much memory your computer has. It is about how the game was developed by EA.
Just imagine you have a 2 litre bucket and you have 3 litres of water and you want it all to go into that 2 litre bucket but when you try to put it in there it reaches a point where some flows over and you get unhappy so you buy a bigger house and have another go at putting 3 litres of water into you old 2 litre bucket and then wonder why it still won't work because your house is so big now.
The thing is Sims 3 was designed to have just under 4 GB of memory available to it. There is no way to increase that. If your computer has 100 GB of RAM and is running nothing else, the game can still only use that 4GB of memory. So, if your lot gets too complex and the computer needs a bit more memory to process something while it's saving then, like the water going into the bucket that overflows as soon as it passes the 2 litre mark, the second the computer tries to use that little bit extra memory, even for a split second, you get an Error 12. This is why I try to find an empty place in my world when I'm saving as I've noticed this usually reduces the amount of RAM being used by the game. It doesn't work 100% of the time and I do still get Error 12 occasionally, but I'd say it works for me at least 98% of the time and I have now got a really good idea of when it isn't likely to work before I even try it to see if it does save. Sometimes it surprises me and saves even though I was sure it would crash on me. Fortunately, none of the games I'm playing at the moment for my videos are giving me problems with Error 12 and it's been a while now since I've seen it happen but I do still go off to a quiet place in the world to save most of the time just in case.
The only time increasing the memory your computer has will assist your game is if you have less than the 4 GB the game can use available to it. You need to have enough RAM for your operating system, background processes and still have about 4 GB for the game.
The big thing mods have done for me is to allow me to play families again. I stopped playing families because the family tree would break. I remember once having a small family with twin toddlers. Then suddenly one of the toddlers was no longer in the family tree and showed up as a stranger to its mother. That sort of thing happened to me over and over and over again until I got so frustrated that I stuck to playing households of 8 unrelated young adults and just watched what they got up to autonomously for a while then I'd make another household of 8 young adults and build them a huge house full of stuff to see what they did with it. Then I'd get tired of that and do it all over again.
Once Sims 4 was released I decided to try mods and now I can repair my family trees so I can play families again. I can even add extra sims to them if I want to. And I've found all sorts of other things that I like to have in my games. But before Sims 4 was released I was mod free and I played like that since June 2009. Except - after having huge problems with Isla Paradiso I put in Error Trap and Overwatch but only had them in my game while I was playing in Isla Paradiso. If I was in any other world I was without mods.
I really should be asleep now - it's almost 1 am.
Happy Simming