I'm currently mining YACoin (a Scrypt-jane coin, so it's a bit different than normal Scrypt coins like Litecoin):
GPU: | Hashrate: | Power consumption: | Cheapest Price: | 7790 | 57 kHash/s | 50 Watt | € 102 | 7950 | 94 kHash/s | 200 Watt | € 226 |
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The 7790 has more than 60% of the hashrate of the 7950 at only 25% power consumption and costs 45% of a 7950. So I'd say the 7790 is the smarter choice, you just need more cards
I will check with Litecoin in a minute.
Edit: here is the result for Litecoin (just standard BFGminer settings, you probably can squeeze out more):
GPU: | Hashrate: | Power consumption: | Cheapest Price: | 7790 | 213 kHash/s | 50 Watt | € 102 | 7950 | 469 kHash/s | 200 Watt | € 226 |
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Ok, with Litecoin it's only 45% Hashrate of a 7950. So the hashrate/price ratio is equal, but the power consumption of the 7790 is still better.
If price and power consumption doesn't matter, and all you want to get is the max hashrate per PCIe slot, then the only choice is 7990. That beast is two 7970 chips on one card and should give you 1200 kHash/s.