This is the kind of set up I started with 2 years ago.
I was running Reason 3 with Cool Edit Pro and I quickly realised I had to upgrade.
I'm now running Nuendo 3 and Sonar 8 with Reason 4 and loads of other apps. and plugins. But in the next few months i'm going to upgrade to a quad core set up.
The best 'studio' PC's will use a quad core CPU with big Caches and at least 3GB RAM.
DAW's like Pro Tools, Nuendo, Sonar will rely on moving a lot of data around a system. So fast buses and big caches is what you need.
Also you want to use 7200RPM hard drives as your audio drives and make sure they have high buffer sizes.
Having said all that you will be able to run Pro Tools and Cubase on your set up but you'll find yourself waiting for your system to respond to simple requests. And as long as you use a decent audio interface your latiencies shouldn't be to high.
So be patient.