Go with a Haswell processor if you can…if not, any i7 is perfect for your needs.
Since you don’t use heavy VI’s, go 16GB Ram with a MOBO that can handle more later on if you need it.
Video card? Most cards now have dual outputs and will do the trick. Heck, you can get a good card for around $100 that will suffice.
I highly recommend Win 7 Pro, running 64bit. That being said, you should also run C7 in 64bit, Omnisphere will love you for it.
I highly recommend a Steinberg UR22 interface as you “soundcard”, it just plugs into a USB port. Mine is awesome, and it is a great unit for $150. And believe it or not, I work professionally with this interface.
Just keep your samples on a secondary drive. In your case, 7200’s will suffice. SSD’s come in handy if you start getting into larger, complex sound libraries like Hollywood Strings, etc.
Go with at least a 600W power supply! It is the lifeblood of your system.
Including the inerface, you should be able to have the system built for well under $1500.