http://www.netcodepool.org still has a Getwork node, and according to the stats, 15% of the users are still using it.

Unless the algorithm of bitcoin is compromised, showing your bitcoin address (like the one in my profile) to people should never be able to result in the coins within it being stolen. You only need to protect the corresponding private key with your life.

For a Radeon 7950, that's the best price I can also find among the different sites.

If you're doing one pool, you only need one instance of cgminer running. Period. Doesn't matter if you're running 1 card, 3 cards, or 7 cards. The only real difference is if the cards themselves are disparate models and require different engine settings, memory settings, intensity settings (which, BTW, you can STILL get around by running one instance of CGminer and using comma commands.)

There are only a couple cases in which I can think of you might need multiple cgminers running on the same computer:
-You want each card to mine a different pool
-You want one card to mine scrypt, another to mine SHA256 (which is a variant of the "different pool" item)
-You want cards 2 and 3 to mine full out, and card 1 to mine desktop mode speed (essentially, intensity 9 instead of intensity 18 or 19)... and want to be able to just flat out stop card 1's mining when you REALLY need the power on a desktop app.

If you don't specifically tell cgminer to use device flags, like -d 1 (to use card 1)... it will use all devices available (1,2, and 3).

Bottom line though: To me, it makes no sense trying to split apart cards unless you have a REALLY REALLY good reason... it just adds complication. Remember the KISS principle.
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I'm still going to say 4GB RAM is VERY low for 3 7950s, and I'm still going to suggest 8GB RAM (meaning you will need a 64bit operating system... not that hard to get nowadays). I'm also going to say 750W for 3 7950s is asking for trouble, since a 7950's max TDP is 200W, your core component's power usage is about 110W in practice... meaning you are already at 710w. First hand experience with a Kill-a-watt and also seeing forums suggests that the cards actually exceed that TDP, and some forum claims say they can go as high as 250W... each. Getting an 850 is going to give you a system that has no expansion as a miner... which is ok for a first miner, but not good if you wanted to add a fourth card.

First hand experience with 7950's is that if you don't have them tuned right but use some of the suggestions off the litecoin mining hardware comparison, you get about 500-550... but many people say if you tune them right, you get 600+. I'm trying to tune my multiple Gigabyte 7950's rig with three of the same card, but purchased a month apart, and running into compatibility issues where they aren't stable above the 550 mark.

BTW, people have suggested you don't want to mix different manufacturer's of cards, nor do you mix different model numbers (eg 7950s and 7990's in same rig) or different generations (7950s and 6970's) in the same rig.

As to who to buy from... I've usually found tigerdirect to be overpriced more often than not compared to Newegg... and even Amazon sometime beats Newegg on prices at times... all while looking at the same exact item.