You are overly optimistic and you are rounding all costs down and all benefits up.
Ebay 5970 prices are rather $450 than $400. You would need a better case (or build one yourself). This one will not give you enough airflow. You need way more ventilation and a few good fans ($15-20 each). If not, you will not be able to overclock the cards, at least not to all way up. The system will not need 294x2 W but 700-750W depending on the PSU efficiency. Depending on your climate, you will need to air condition this 700W. The BTC bid price is, last time I checked, $0.87, not $1.00 and nobody knows, where it is headed (it was $0.6x just a few days ago). And if it is indeed $1.00 or more, you can be certain of a significant difficulty increase. Before you even order these parts, the difficulty is going to be 10-15% more and when you have it all running, you will be half way to another 10% of difficulty increase. Taking all that into account (especially the difficulty increase of 10% every 2016 blocks which may be conservative but during all my time with Bitcoin, difficulty increased faster than I always expected), you are going to get 1500 BTC before electricity costs in 180 days (minus 3 BTC a day of electricity costs with AC=540 BTC) which is not going to pay for this system at current BTC prices. Then, another 90 days will bring you only 300 BTC before electricity (but maybe the difficulty increase will slow and it will be a bit more). Did I mention that your eBay card may fail after a few weeks of constant use and you are out of warranty?
In my opinion, it is well worth mining with a system you already have. And it is also worth building a mining rig if you like building computer systems and monitoring it because it's fun. And the mining will eventually pay for the system (but not for the time you spend on it so consider it a hobby). But profit? Not so much unless the BTC price increase well beyond 1$ (and better yet not now but later so it does not impact the current difficulty). And for the prices, we are in the "tea leaves reading" territory. And if you expect higher prices, it is much simpler to just buy BTC, instead of the mining rig.
The dedicated mining rig is the riskiest option out of the two that I posed. The fact that this market can gain/lose 40% of it's value in the course of a week is insane, and makes it a risky investment. I absolutely recommend just upgrading, as you have less time (and therefore less risk) until you hit the break even point. And even then, you can "get out" at any time by just selling the card itself.
Trust me, I don't have the capital nor the balls to build a dedicated mining rig. I have upgraded two computers and nearly have them paid off (sold off all of my BTC in the last day). This just shows that it is possible to make a short term profit mining bitcoins. Is it a risky investment? Hell yeah, as there are a number of things that could go wrong. Is it safer than speculating in BTC itself? In my opinion, absolutely. I would rather put $1000 into a mining rig and have the fallback option of selling it if the market tanks, rather than owning a lot of worthless currency.
And to all of the people attacking me on difficulty: can you recommend a way to "guess" at a better break even point using a difficulty estimate? I'm not concerned with anything past the break even point (~45 days), as that is when it becomes purely profitable to run your rig regardless.
And also, yes, I get 370,000 khash on my 5870 on air through overclocking. The average between my two currently is about 360000 khash, and the hottest temp I have is 70 degrees celcius.
