I recently bought a Phenom II system with similar intent, so I can at least tell you what I suggest from the research I did for my system.
As far as memory, I would suggest going for 4GB of DDR3 nowadays. It's reached price parity with DDR2 and in the future DDR3 will get cheaper while DDR2 will increase in price by the time you want to do a memory upgrade, Buy 4GB of DDR3-1333 as it's about the same price as DDR3-1066 anyways.
I definitely suggest Radeon 4XXX cards simply because of the 7.1 LPCM that you can use with an HDTV or a receiver if you have one or plan to get one in the future. AS far as what exact models, you probably have DVI monitors which eliminates most of ATI's low end 43XX and 45XX cards which only have 1 DVI and 1 VGA output. Therefore, two Radeon 4670's would be what I would suggest.
I would definitely recommend an AMD 790X or 790FX chipset as you already have the two graphics cards so if you ever need the 3D power you can use CrossfireX. The 790X will only give x8 bandwidth to your graphics cards compared to x16 with the 790FX, but it won't matter if you don't do much gaming. I have good luck with gigabyte boards, so I would recommend the 790X-UD4P, but it's really a personal choice.
For CPU's, any AM3 Phenom II should suit your tasks. I recommend the X4 945 as it seem to fit within your budget, but the X3 720 is also a great choice.
As far as case and power supply, I bought the Antec NSK6580B and would recommend it to others. It comes with a 430W power supply which should be more than enough for your needs as well.
From what I found on Newegg, this comes about $640 before three $10 rebates, leaving room for additional upgrades if you want more memory of other items. Good luck with your purchase!
My current system (purchased before AM3s came out):
Phenom II 920
2 x ATI 4350s (for 3 VGA monitors)
4GB OCZ DDR2-1066
Antec EarthWatts 430W PSU
Antec NSK6580B Case
6x Seagate 500GB 7200.11 RAID 5
With crossfire, you can't use more than two monitors at once. Therefore, you'll have to enable it when you want to have the improved performance and disable it when you want to use 4 monitors. Fortunately, this is done in software so you don't have to change any bridges when you want to change from crossfire to 4 monitors.
I have not overclocked my system - I don't want to risk stability and it's more than good enough at its stock speed for what I need it anyway. I'm just using the stock AMD HSF, and it's quiet when it's set to auto fan speed.
Why do you want to go with 2 ATI 4770? They're mainly gamers cards, pretty useless to you, stick with something simpler, like the 3450... You just HybridCrossFire this card with the board's card and you get: 3xDVI, VGA, HDMI, S-Video...
Also stick with G.Skill ram, its' cheaper and better than the OCZ you've found.
The 790FX chipset in your case is overkill, at 180$ this board is too much for you, get a simpler (look at mATX too) 790g chipset board like the Asus M3a78-em.
About RAM there's not much reasons to go with DDR3, except future upgrade, but this system will serve you well for the next 2 years at least using it as you've pointed out.
For the CPU I would say don't go BE if don't plan massive overclocking (and if you don't forget getting a HFS, I recommend Zalman's CNPS9900; awesome performance). I would go for a 710, usually cheaper and the differences in performances are near 0% without overclock.