1. So I`m currently mining on my AMD 5970 and it gets about 750 m/hs. Is it really worth buying more gpus or mining computers in the long run? Do I really get enough bitcoins to cover my expenses in the future?
Continuing to run currently owned mining hardware is definitely good, as long as your electricity costs are covered. Purchasing new mining hardware is... not so clear. You might make a profit, you might not even break even. Make sure to take difficulty increases into account when doing any calculations.
2. If I were to buy a new mining box, what gpus should I buy? I would only be able to fit 2 cards and not the most expensive ones e.g 6990`s
5970 is still the top dog. As far as I know, it hashes faster than the 6990 and it more efficient.