I am currently running 24GB since I have a 1st Gen i7 and a Titan X and memory has never been an issue. Only issue I have had with UE4 lately is a lot of programs in Windows 10 use the same things and if opened lags UE4 but that’s just Windows and software competing for a spot. If you can afford it and your system supports it go for 64GB not going to hurt anything gives you plenty of wiggle room. Hell with 24 I can have UE4 open, multiple YouTube vids going, stuff running in the background and I maybe hit 24-50% usage. Only time I had a RAM issue is when loading up one of the large tech demos for the first time though that could also be cause my CPU is starting to show it’s age.