its already peak down

oh yeah i was considering an rm850 would it work with a rtx 3090

any psu since i have 650w rn

RTX 3090 (zotac) 500 cad - 358.62 usd
Rx 7800 xt (founders) 600 cad - 430.35 usd
and why?
both used (note this is the cheapest rn and it wont go down on facebook marketplace)