If it’s PCI Express (as opposed to regular PCI), then it pretty much should work.
What may happen however is that the slot will run at a slower speed, so if you put a 5090 with a Core 2 Duo you will struggle to keep the GPU fed with enough data to fully load up the GPU while your CPU is pegged at 100%.
It’ll run though.
EDIT: You can also have issues with the legacy BIOS and your newer card not shipping a BIOS ROM to initialize it on boot, but once it gets into the OS it should activate. If you have an iGPU it should output there until the OS starts.
Ironically, high end cards usually only have small penalties for having a previous gen PCIe. On a full x16 lanes, there’s usually bandwidth to spare. Although you wouldn’t want to put a RTX 5090 on PCIe 3.0. It was the RX 6500 XT with its mere x4 lane count that took a significant hit running on PCIe 3.0 instead of 4.0. That was not a good limitation for that market segment since the low end is where people are most likely to try to avoid upgrading motherboards/cpus.
If it’s PCI Express (as opposed to regular PCI), then it pretty much should work.
What may happen however is that the slot will run at a slower speed, so if you put a 5090 with a Core 2 Duo you will struggle to keep the GPU fed with enough data to fully load up the GPU while your CPU is pegged at 100%.
It’ll run though.
EDIT: You can also have issues with the legacy BIOS and your newer card not shipping a BIOS ROM to initialize it on boot, but once it gets into the OS it should activate. If you have an iGPU it should output there until the OS starts.
Don’t forget the PSU. You go with a power hungry gpu, and you have an older psu that’s say 400w, you’re not getting clean power.
Ironically, high end cards usually only have small penalties for having a previous gen PCIe. On a full x16 lanes, there’s usually bandwidth to spare. Although you wouldn’t want to put a RTX 5090 on PCIe 3.0. It was the RX 6500 XT with its mere x4 lane count that took a significant hit running on PCIe 3.0 instead of 4.0. That was not a good limitation for that market segment since the low end is where people are most likely to try to avoid upgrading motherboards/cpus.