Thank you all for responding !
I started looking at the 7990 but it is NOT very easy to find here in Holland. Even on Ebay in other countries you don't find them for a bargain.
I mean, they are 4 year old cards and eventhough they don't say it thet probably mined the shit out of them.
1080TI would be great but way out of my budget, they are asking crazy prices for them these days.
Would 2x crossfire HD 7970 make any sense ? These i can find at a good price.
Crossfire is not necessary for mining, and can often reduce performance and/or create a number of different problems.
7970s were once the flagship single-GPU cards, but as always, whether or not they're worth it largely depends on the price you can get them for. Back when GPU mining BTC and LTC was profitable, 7950s were actually more popular since the 7970s usually didn't offer a performance increase that justified the additional cost & power.
However, you'll want to check your options carefully. The GPU mining scene today isn't exactly the same as it was with BTC/LTC. Now, the amount of graphics RAM is more important. It's only a matter of time before 3GB cards become obsolete for Ethereum. (There are 6GB 7970s floating around, though.)
Check out hashrates for mining performance on other cryptos, such as Zcash.