As you may know, Ethereum mining is so profitable atm that its nearly impossible to buy any new GPU's.
There are used gpu's on ebay but the prices are reaching that of brand new cards (which you cant buy atm)
If you want to mine Ethereum you need a GPU with more than 4GB memory.
You can still mine some coins with 3GB or 4GB but they are less profitable.
Use this calculator to see what can be mined with less than 4GB
Dag size calculator
Currently the best cards to buy new when available (next estimate is Feb 2021)
Are the RX 570 8GB or RX 580 8GB
The RX 570 is the best power usage to hashrate for the best price.
Nvidia cards can be used but they are more expensive for less hashrate.
Unfortunately not all cards are equal, some brands work better than others. Also its sometimes a lottery as to which brand of memory is in your card. AFAIK Samsung is the best.
If you are looking to buy 1 then i recommend googling that specific brand to see if you can find its hashrate. Some have a poor hashrate straight out of the box.
However, if you are prepared to find a mining bios, then most cards can hash at the higher rate when modded.
Heres a link to different brands of 570.
RX 570 comparison
The best of the older cards are :
RX 470 8GB
RX 480 8GB
Vega 56 8GB
Vega 64 8GB
R9 390 8GB
Here is an extensive list of all GPU's capable of mining, via the minerstat website.
hashrates in the links.
Ive noticed some of the hashrates are not correct so just use this as broad guide.
e.g. I have 3 x Vega 56's and they all hash at about 43 MH/s each and use around 145 W when using optimum settings
whereas the vega 56 on minerstat is listed at 34.74 MH/s, 193 W
Minerstat GPU's list
Here is another GPU mining calculator. fill in how many GPU's you have in the boxes then click on the GPU.
What to mine
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