A friend has an i7 5820 (64GB RAM) and asked me if she can use one RTX 4090 or two?
Is a new CPU necessary for GPU rendering?
I think it depends on the motherboard and not the processor. I could be wrong.
It will work fine but it might not be working as fast as on a modern CPU. I think @Muhammed_Hamed might have some more concrete numbers but if you’re running it on an old(er) CPU there is some performance you’re leaving on the table. From my albeit limited understanding it shouldn’t really be that much but then again if you’re buying it for speed and not VRAM it might sway you to go with a 4080 and save a couple of bucks.
With that setup you’re also going to be on an PCI-E gen 3 platform which is I think again fine but there’s a little you’re loosing there as well.
Like what @nkilar said, the CPU you intend to use is 10 years old
You will not get the full potential of your 4090s, specially with new features like the new Out Of Core Textures in 6.2 which relies on PCIe bandwidth, it will perform better with PCIe Gen 4 and Gen 5
These GPUs are very expensive anyway, you might as well invest like 1000 Euros into a good CPU and motherboard for them
Best,
Muhammed