Thank you for that but in the mean time I was so frustrated by the "check the calculator" answer that I googled "litecoins MHs 6950" and eventually found out the MH/s value for my Radeon 6950 2GB.
So I'd be doing 350 MH/s in Bitcoin and 0.42 MH/s in Litecoin. That would be $1.068 net profit for Bitcoin and $2.178 in Litecoin with electricity at 0.10 $/KWh. But here's the funny thing. I have two Radeon 6950 (I only used one in the calculator). And I think one of them doesn't crash with the 6970 bios. So when I hit 710 MH/s I hit parity. Except if my electricity cost in not higher. Because if it's 0.12 $/KWh then I still won't hit net profit parity with Litecoin.
So what do you recommend me?
Test it out. Your cards will likely differ from the hashrate values listed for the cards. Download a miner, preferably one that supports both SHA256 and Scrypt, make two copies of it (one for BTC and one for LTC), configure a mining pool for each, then test out each one to see the real-world hashrates you get for BTC and LTC. Do the test for the 6970 bios as well. I personally would think that with the real-world values, you'll still make more from mining LTC than BTC. Just note that the prices of BTC and LTC, as well as the ratio between the two, are EXTREMELY volatile and swing dramatically each day (especially for BTC). Just this month alone, BTC has adjusted from over $250 per coin down to $100, then down briefly to the $60-$70 range, and has ended up back up in the $90 range today. LTC was at $4.50 and is now down to $2. In truth, it is very worthwhile exploiting the volatility of the market while mining away the most profitable currency.