nakowa, HD 6990 is not the best card for mining
too expensive. a cheaper 5xxx series will get you more Mhash for your $ AND use less power and run cooler
so you seem far from an 'expert' to me with your naive advice to buy expensive gear.
Really? Which cards do you recommend because I was planning to buy one 6990.
The 5870 and 5970 are the more energy efficient (cheaper electricity bill). The 5850 and 5830 are usually the best for initial cost per MH/s but it really depends at the price you buy them (and you have to think that if you plan in selling it at some point, the series 6 will be easier to sell than the serie 5, but this is only important if you plan on selling).