First off, the CPU and memory are irrelevant. I am running 2 machines full time right now, and I spent $30 on the processor and $25 on the memory.
Take a look at this page:https://en.bitcoin.it/wiki/Mining_hardware_comparison
It will give you a good idea for each graphics card what the hashrate is. If you aren't overclocking, you can reasonably expect a 340 Mh/s rate for a total of 680 Mh/s.
Then, go to this page:http://www.alloscomp.com/bitcoin/calculator.php
enter in your hashrate and the exchange rate (I use 20$ / BTC since I won't sell lower than that - but it is always speculation)
You can reasonably expect around $474 / month. That is at the currently difficulty.
Now, go to this page:http://bitcoincharts.com/markets/
You can see the next difficulty is estimated at 1114107 and should change later this week in 1082 blocks. You can divide the blocks till change by the current block / hour rate (9.49) to get the estimated hours until the change (114 hours / 4.75 days).
I like to do my short term calculations on the next difficulty, and I figure that would be a good average for the next month. Going back to the mining calculator, and putting in 1114107 nets a monthly return of $374.
Now, you also need to add in electricity costs. You can estimate it out by calculating the watts your video card uses (see mining comparison) and multiplying it by .72 to get the kilowatt-hours. Then you use your electricity rate to calculate your electricity costs (mine is 8.1 cents per kwh for comparison, I have heard of people with up to 15 cents per kwh).
If you want, you can also pad it a bit for cooling costs since the machine puts out a lot of heat. I recommend finding a good solution to minimize that. I have a thread with my setup, a fan with a garbage bag funneling the heat to a vent and out a window.
The other consideration is solo vs pool mining. If you solo mine, you won't get your first block for around 8 weeks (though it is completely random, could be first night). When you do, you'll get 50 BTC, worth about $1,000. With pool mining, you lose a couple pct and all transaction fees, but you get steady income. Unless you plan to get a lot more power, I'd recommend pooling.
Edit: redid my calculations when I realized you had 2 video cards