Noise is not really a problem but how loud does it get? and also I get to test the card before i buy it. so what do I look for to know if it is "sacked" or not
Hm, it's kind of hard to tell tbh. If you can test it before buying, run some stress tests like Unigine Valley and FurMark.
You can look up the 7990s Benchmark scores for the Extreme HD Preset in Valley and see if the results match.
Or while you run Furmark, you can look at the clockspeeds and temperatures in MSI Aftrerburner and see if everything is boosting up niceley.
And then you can of course look at the cooler, do the fans run smooth or is there any rumble from a bearing or sth.
With that being said, the 7990 is a high performance card It is, as mentioned above a combination of two HD 7970s (which is the same as a R9 280X) on one card. As a result of that it should outperform a R9 290X or GTX 970 in most games, however at the cost of being power hungry, loud and hot.
I personally and many others recommend one strong single GPU over two smaller GPUs in Crossfire or SLI (Which the HD 7990 inevitably is).
So in other words: if you need a Graphics card now, and you're kind of on a tight budget, the 7990 is worth a thought. If you howevery still have an okay-graphics card and you can wait a little bit, i would recommend to either save up for a GTX 970 or look into what AMD is releasing at the end of Q2 this year.
I guess a used R9 290 or 290X would also be an option, as they are still good price/performance crackers! (however the same argument about cryptocurrencies applies here) The whole dual-GPU thing is what puts the HD 7990 a bit off.