When you play modded minecraft like this, it is fairly demanding, mainly because it is done in java (very bad solution for gaming) and there are tons (200+) large mods in a good mod pack.
You want to have at least 8GB ram, preferably 16 to have reasonable performance and a decent GPU.
Yea, I created my own modpack with something like 56 mods, and I have 8 gbs of ram, and I had to close the game and reopen it because of the memory leak lol. Though when I was playing on my server it wasn't that bad.
All the Mods 3 is what we play now. It's a kitchen sink mod with a ton of mods and fairly bleeding edge.