Since you already have a 3060, I would recommend against upgrading to a 3090 Ti when the 4080 is around the corner. Even with the lower prices of a 3090 Ti, this will likely be near the MSRP to a 4080. The 4080 should still be significantly faster than the 3090 Ti.
As for VRAM limitations, the 16 GB of VRAM on the 4080 is more than enough. The 3060 Ti is a bad comparison for VRAM, as it’s usuling slower GDDR6 memory modules and requires high capacity versus GDDR6x memory.
On a side note, you will need a bigger wattage PSU regardless which card you get. Going with a quality 1000W is a good place to start in terms of value and cost. I would not cheap out on PSU or try to run with your current PSU, as this can cause headaches with high power draw cards.