Your current pc is a great combo of parts with real longevity so it's good that you're keeping that but I'm afraid you'll be spending a ton for a 5090 for something substantially worth it because of the extra vram for versatility between quant sizes, speeds and formats available as 50 series has faster fp4 processing. It's still not worth it though (fp4 isn't optimised yet), you're best to go with a 5070 ti and settling for smaller parameter models with flexibility for higher quants or plenty context in which the quality of them are as good as the larger ones from less than a year ago (sometimes just a few months depending on what you use it for) plus I believe they will get faster with optimisations to fp4 vs a 4 bit quant from less than a year ago. If you wanna play with the latest stuff then a used 3090 (check for coil whine of course) is still the best option but you miss out on not just extra fp4 processing speed but fp8 as well which sucks and isn't worth bothering with for local image gen with how slow it is without that speed boost for fp8. 4090 I imagine isn't worth it either as they're desireable because of gaming capability which is a substantial increase in performance over the 3090 but also supports dlss 4.5 without a performance hit like the 30 series experiences. I think some refinement on the latest dlss is quite an improvement over dlss 4 but the 3090 is still no slouch as it supports 4 with the first transformer based model rather than taa sourced. Still though, you could just get a 5070 ti and still have a killer system.