Yes but in turn a lower end motherboard may not last out long. ASRock are not that bad manufacturer, I would definitely read the reviews first on it. Or go for a lower end ASUS or Gigabyte model. It looks decent but it isn't going to produce as good results as the ASUS or Gigabyte in terms of performance and quality.
To be honest 8GB RAM should be plenty, so I do not know what programs you were using to full up that much. You have to remember DDR3 RAM is much faster than DDR2.
SSDs are fast, they're chips, they use PRAM (Phase memory) correct me if I am wrong or are going to use Phase-change memory. Either way they are faster and live longer. But to be honest I agree with you, I don't think getting an SSD will be worth it, Yes hard drives do have more problems and decrease in performance over the years, but its cheaper to buy a new HDD than a SSD. I would get an SSD when they drop in price by a lot.
And OCZ are good power supplies (I don't know about the other models but the ZS is decent). And cheaper in PSUs is not the same case. Cheap power supplies are cheap because they cut corners with voltage regulators, heatsinks and quality components and capacitors. That's why you can usually judge a PSUs quality by its weight (not in all cases though). OCZ would be fine, but in the long run you may find a Corsair would be still running strong and stable compared to the OCZ.
And you currently have 2x HD 6850s? Well in that case if they are both in good working order you have no reason to not use them. I thought you were going to buy 2, in that case yes it'll be cheaper getting a decent power supply
Make sure they are in good working order, because you mentioned a display not being supported. Sounds like driver issues in any case. Mind, running at 1280x720 is very low, so you will find a single HD 6850 will do it fine. You only really need 2x HD 6850s if you are planning 1600x1200 and higher with full AA. HD 6850 could max out anything on your current resolution.
And its the way the architecture is, more throughput, using a 28nm design. But remember using crossfire doesn't mean 100% more performance, because it has to pass over the information using the crossfire bridge. Also in crossfire your cards bandwidth will be half, running at x8 rather than at x16, so processing is going to be slower. You may only get another 50% performance due to this.
And on air to be honest most branded cards such as XFX, ASUS and Sapphire come with pretty decent heatsinks to allow for overclocking. The HD 7850 can be overclocked around 200mhz on air. My HD 5830 has been overclocked 200mhz on air and is nice and cool. Voltage increase is usually the way to heat up a card
And a couple of fps making it lag or smooth would mean you have a card on the edge at 30 fps. Over that and everything is pretty smooth. But to be honest if you are going to run at 1400x900 a single HD 6850 will max out anything