Alright. I was making that assumption based on online psu calculators. Also I have read that each cable is rated at 150w and since the gpu uses at most 300w this solution will be good.You need a higher wattage PSU...to also add, you don't have the necessary connectors. Why? you may ask? It's because you shouldn't daisy chain PCIe power connectors to the GPU, they should all be standalone from the PSU's end. The second point being that you lack necessary power from the unit(even if the unit would've been brand new). MSI's own listing for the GPU, here, suggest a 750W unit. I'd add 100W to that number and invest in a reliably built PSU to power your build with the RX6800XT. I'd go for a higher wattage unit than suggested, for the sake of headroom, if I went with the RX6800XT.
I am located in Hungary, I dont order from international websites. If that helps you we have Alza, Emag and stuff. Also our currency is kinda messed up but an estimate would be around 125 euros.Welcome! Where are you located, what is your budget and what is your preferred site for purchase?
Alright here is a list from cheapest (here) to the more expensive from the 750 w psus:Corsair, Seasonic, EVGA and Asus are some good brands to look at. You can provide a link with a shortlist of what you can afford. The community could chime in on suggestions.