Will dual channel memory double my FPS in Valorant with a 5700G?

No, it won’t double your FPS, but the improvement is big enough that you should upgrade immediately. Going to dual channel roughly doubles memory bandwidth, and in games like Valorant, you can expect around a 30-50% FPS boost at 1080p low — a jump from 60 to maybe 90 FPS. Don’t overthink it; buy a matching stick and install it.