You already are pretty much building half a new computer. The sff form factor and psu are hard limits to what you can do. Best gpu you can have is a gtx 1650 sff and it's not cheap as it's an sff card. A 4790 hovers around 100$ at least too so that's expensive. Ram won't be too bad but still around 80 for 32gb. Basically you will be spending close to 400 which is 100 away from a complete new system that will be better than this one. My advice is to get a ryzen 2600+b450 motherboard+whatever amount of ram based system with a used gpu. It's going to end up being the same price and be better than what you can do now.
