Well, for redshift you absolutely need a gpu with 6GB vram. Better 8 or more. So a 3060ti would be a budget friendly start. For sims, you need as much ram and raw cpu power as possible. For getting started, I would say 64GB at least with a current 8-12 core cpu. If your budget doesnot allow for doing both, I would recommend: Upgrade ram and cpu and stick with karma. When you earned money with it, buy an entry level card like the 3060ti. And leave your old gpu in for driving the monitors. This will save you quite a bit vram on your 3060ti. You will benefit from faster preview sims and much faster karma (xpu) rendering. At this level going for redshift will be an option. Alternative budget wise for starting with redshift: Look for a cheap used gtx 1070 until can afford a rtx.