Hello @rainer1973at! Welcome to the Red Team Community!
What is the primary purpose of upgrading your PC? (Gaming)?
That said, I don't recommend "combining" the two graphics cards (GPUs) as they are not the same product (different architecture, the GPU in the Ryzen 7 6700G features our RDNA2 architecture). When looking to "crossfire" two AMD GPUs, the GPUs should be the same.
Personally, I prefer to have a separate, discrete, GPU (not integrated with the CPU). This is ideal if you are planning to play lots of video games.
About your last question, well, the Radeon RX 480 is an old product (it was launched in 2016), and it's difficult to compare an APU (the Ryzen 7 6700G) with a discrete GPU (the Radeon RX 480). That said, I believe the AMD Ryzen 6700G graphics card will outperform the 480...easily 🙂
Sam
Your biggest fan!
CPU: AMD Ryzen 7 5800X3D GPU: AMD Radeon RX 6800XT