Is 8GB of RAM enough for Roblox and Minecraft?

Probably yes for basic play. Roblox and vanilla Minecraft run fine on 8GB of DDR3 with an i5-2400. The lag is more likely the CPU or integrated graphics, not the RAM. Heavily modded packs like All the Mods may need 16GB, but check Task Manager first—if RAM isn’t pegged at 7.5GB+, more won’t help. Turn down render distance and fancy graphics before upgrading.