With a single 7970 what should I start mining right now? Is it even worth it?
Enter your input numbers here:
http://bitcoinwisdom.com/litecoin/calculator
If you are in a jurisdiction with tiered rates, you need to figure out the marginal cost of electricity you will use from mining. For instance if you are on tier 1 but will go up to tier 2 due to mining, then your cost of electricity for mining is whatever your tier 2 rate is. (Tiering is a crude incentive mechanism to get people to conserve energy for environmental reasons as well as cost reasons because it's cheaper to conserve energy than it is to build brand new electrical plants and transmission lines.)
If you already have everything you need for mining (a video card and CPU, mobo, RAM, PSU, etc., then those are sunk costs, so you can put "0" for "hardware price."
Per the "overview," an overclocked 7970 can generate about 700 kH/s at 250 watts at the wall with a typical PSU. Add another 50-100 watts to that if you have a typical CPU, mobo, RAM, HDD, optical drive, case fans, peripherals (keyboard, mouse, etc.).
Pool fee you can probably leave at 2% because even if you mine at a 0% fee pool and don't donate anything, you will get some stale shares and other such downtime.
If you are having a hard time guessing what to input for "difficulty increment" in the calculator linked to above, look at
http://bitcoinwisdom.com/litecoin/difficulty and enter something you think is reasonable based on that. Difficulty has increased by 60+ and 80+ percent in Nov and Dec 2013, and January looks like it might be slower but is already up over 20% with half the month to go so it's on pace for more than 40% difficulty increase. For reference, a 5% "difficulty increment" translates to (1.05)^8-1 = 47.7% monthly increase in difficulty which seems about right to me eyeballing recent trends and considering January looks like it's going to increase in difficulty somewhat slower than Nov-Dec.
Click on your preferred currency which is probably USD (US Dollars).
Click on on the "Months" button where it says "Group by." That gives you the cumulative profit/loss you can expect given your inputs, by month.
There, you are done. This is a good first approximation of profitability. You can potentially make more or less depending on whether prices per coin rise or fall, among other things, and arbitraging altcoins may give you more or less profit as well depending on how well the altcoin market does.
Mining arbitrage (where it's more profitable to mine something instead of LTC) is difficult because opportunities are usually very fleeting due to intense competition. The more hashpower attempting to arbitrage, the faster the arbitrage opportunities disappear which is why coinhopper pools aren't as profitable now as they used to be when they were smaller, especially when you factor in their higher pool fees and higher stales/rejected shares. Even disregarding fees and stales, it's possible for such pools to make less than litecoin mining if the pool algorithm messes up and the pool winds up mining something it THOUGHT was going to be more profitable but turned out to be less profitable because prices changed faster than the pool could adapt. I did a test recently and middlecoin was basically tied with mining at a 0% fee litecoin pool with 1% stales on that test day. The next day it was actually slightly less profitable to mine at middlecoin, don't know if it was just bad luck or what. Altcoin mining has a large hashrate associated with it, so if people mine altcoins it benefits litecoin miners, too, by reducing litecoin difficulty. So litecoin miners get a share of the profits from altcoin mining. Anyway, if you think you can beat LTC mining by X% in the long run, even after higher fees and stales, then give yourself X% more kH/s instead of 700 kH/s for your hashrate, in the calculator above.