Ryzen 5900x with DDR4 or 7800x3D with DDR5?

Hey there, I'm planning on building a pc soon and have settled on using a Radeon 9070xt as it looks like the best GPU for me as I wanna use Linux and dgaf about raytracing. Thing is, I'm split over what CPU to buy and if the extra 300/400 euro spent on RAM would be worth it. My brother has some DDR4 lying around and I'm not sure if the CPU + RAM upgrade would be worth the cost. My overall budget is 2 grand and I have a few parts (SSD and Power Supply)from an old pc that needs to be put out to pasture so I'll hopefully be able to save some money. I live in Ireland 🇮🇪

There's probably no right answer to this but some feedback would be appreciated :)