If you are looking to future proof this and you happen to live near a Fry's Electronics or MicroCenter, I'd suggest going with i7-930 cpu ($199) and the Gigabyte x58a-UD3R motherboard. That is going to provide more upgrade paths in the future, including support of another 5850 if you wanted.
If you were to go the i7 build, I'd suggest going with G.Skill Pi 1600 ram (triple channel) as it's very good ram for a good price.
I'm not too experienced with the Western Digital C B HDDS- I usually stick with Seagate 7000.12- they're pretty cheap, just as effective for gaming.
I would look into the Coolermaster HAF 922 case- it's very cheap, usually has combos with the above mentioned HDD, and is one of the best cases out there (For the price- performs excellently on highly overclocked comps).
you could go with a core i7 build but for gaming is not much difference to a amd phenom x4 955 BE build but you will be spending more money. I did read somewhere that at microcenter has i7 for 199.99 which makes a very good buy but if that deal doesn't exsist any more then i would go for amd build.
the mobo that would go with the 955...first mobo is a newer mobo
HAF is a good case, but on the other hand, if you don't want to spend that much on a case you can go cheaper - I don't see your system having heat issues especially if you are not OCing.
i5 and i7 sockets will be obsolete in a year anyway so which you go for doesn't matter in tems of future proofing - both will last you 3 years or more at which point all sorts of new technology will be out. Of course, having said that, if you can get the i7 for $199.99 then there is no harm in grabbing an X58 mobo.
The best P55 mobo is the Asus P7P55D E Pro, but it is expensive at $189.99 an iif you do not intend to XFire at any point it may be worth buying less.
As for differentiation between kit and RAM, everyone knows what is being refered to when someone says dual channel RAM or triple channel RAM - whether you call it triple channel kit or RAM the fact is the same - it won't work on a mobo designed for Dual Channel