Cubase 4 is really a good joice. If you're still student (on music collage, university, etc.) you have even the benefit of "education prices" from Steinberg (at least here in Austria/Germany) it is possible. And you pay only about half of the full price!
But if you just want to start there's also a light version of Cubase 4 called "Cubase 4 Studio" if I remember correctly. Depends on your needs and budget.
About soundcards/interfaces: I can recommend you RME hardware too. It's a german company, www.rme-audio.de with a big sortiment of all kind of hadware interfaces with high quality! Especially if you're planing to go on road. They have many mobile solutions, e.g.: Multiface, Fireface 800, or Fireface 400, etc...
Temporary my setup is: AMD X2 Dualcore 4400+, 4GB Ram, Cubase 4 and RME Multiface. The Multiface has Midi In/Out, 8 analog In/Outputs, digital (optical, coaxial, ADAT) wordclock, etc.
The only disadvantage ist that you need to buy a seperate PCI Card that doesn't come along with the Multiface unfortunately.
As you're using Windows there's no possibility to use Logic or DP, but Protools. But instead of buying the cheap Protools with M-Audio it's much better to go with Cubase (or Nuendo) and RME hardware! Unless you have really a huge budget, then I would recommend you definately to buy a professional Protools System. Or instead a Porsche or a villa in Malibu :-) (just kidding)
Good luck,
Andie