1600 Mhash/s - 1x7950, 2x 7870, 1x5770
That's impossible. What you're getting is probably 1600 kilo hash which is only 1.6 mega hashes which is nothing.
I'm shocked at how 90% of the people here can't tell the difference. 1,000 kilo hashes is just 1 mega hash. The best ATI card at frys, the 7970 (they don't yet carry the 7990) only does 600-700 kilo hashes. And when you run these cards together you lose efficiencies.
The 7970 does 600 to 700 megahashes/sec for BTC mining. For LTC mining you will get roughly 600 to 700 kilohashes however.