8GB is probably enough for Roblox and Minecraft.

Probably not. Roblox and vanilla Minecraft run fine on 8GB of DDR3 with that i5-2400. The lag you’re seeing is more likely the CPU or integrated graphics struggling, not the RAM.

Minecraft starts to chug when you pile on mods and shaders. If your sons are running heavily modded packs (like All the Mods or RLCraft), 16GB might help. But if it’s just base Minecraft or light mods, 8GB is fine.

Check task manager while they play. If RAM usage is pegged at 7.5GB+ and the CPU isn’t maxed, then sure—grab another 8GB stick. But if the CPU is pinned at 100%, more RAM won’t do anything. And if you don’t have a dedicated GPU, that integrated graphics is the real bottleneck.

16GB DDR3 sticks are cheap used—like $15. So it’s not expensive to try. But don’t expect a miracle.

Focus on turning down render distance and turning off fancy graphics first. That’ll buy you more than a RAM upgrade will.