You planning on using Linux or Windows?  If it's linux, then flashing the card bios is probably your best bet.  Windows, I'd use something like MSI Afterburner or Trixx and set it to autostart.  Though since these boxes have to run without any supervision, I'd go easy on any overclocks just to keep the cards as stable as possible.

You may want to set the box to automatically reboot itself every couple of days just to clear up and hung up miners, etc.  If you can set this in the BIOS, all the better in case the OS is hung up.

As for the power question, you should be good with either PSU on either machine.  Both those CPU's run at about 95W.  As for the cards, the 5850 is about 150W, 5870 about 200W and the 6950's are about 175W each.  (If you overclock a lot, that'll go up).  So, the 850 will have more than enough headroom for either setup.  You could even underclock the CPU's a bit if you want to save some power.  

Couple other things I can think of.  Make sure you have your miners setup with a backup pool/miner in case the primary one goes down.

Also, you might want to get this up and operational a week or so before you plan on heading out and let it run on it's own so you can shake out any major problems.

You can easily set up your miners to auto start on login through either the miner itself, a windows task or even showing the miner shortcut or a batch file into the Startup folder.  Of course that's using windows, and considering you want to keep the machine running 2 months straight, you might want to look into linux.  Either way, to keep the machine stable, you may want to go easy on the overclocking.

Also, most machines nowadays have a BIOS setting in the power settings that you can tell the PC what to do when it recovers from a power outtage.  There should be an option to turn the machine on when power returns.  There are also devices out there you can buy to cycle power remotely if you really want to deal with that in case windows locks up and isn't responsive.

Finally, assuming you have internet connectivity for those machines, just use Teamviewer or VNC to remotely connect to them.

I'll sell you www.bitcoindomainsforsale.com - it could help you sell yours.

these aren't as bad as I was led to believe by the first reply. I like a few of them for sure. good luck.

I don't think you can compare a design with a domain.
If you have 2 options to give a visit since your looking to buy cars.

Option 1:
buygoodcars.net

Option 2:
buygoodcars.com

Were will you buy?
The answer is: You cannot tell yet, since you didn't view those websites yet.
Like said, if you think buygoodcars.net has a good design and cousin Evan told you buygoodcars.net sells great stuff.
Why then say: Hey but wait a minute... buygoodcars.com has a .com extension. I should buy there.

No, just no.

blocksolvers is the only good one
some are outright embarrassing, like INVESTINBITCOINS

Hi there...new the the forums and just picked up a 5830 and a 600w PSU to go along with it -- they arrive Monday, and I'm excited to really get started. I am not as tech savvy as a lot of you guys so I'll be getting a friend to come and help me install these pieces (surprisingly, the power supply is what I'll need help with -- I'm awful with cords)...I did manage to already set up CPU and GPU miners on my own, though, so I'll probably be fine once I get the hardware out of the way. I'm using an nVidia 9300GE, which is CUDA in name only. Between the Athlon Dual Core 4850e (not OC) running at ~2.5GHz each and that terrible card, I've been pulling down about 3.5 mhash/s...which is pitiful. I make laughable amounts of BTC right now. I figure I can turn the CPU miner completely off once I get this card installed (no point in running it, really). Also thinking about slaving the gf's parent's computer, once I see what hardware it's running. Maybe I'll see a whole bitcoin one of these days...

Anyway, looking forward to being an active part of your community. I'll come and let you all know how everything turns out once I get the card installed.