You'll get alot of differing opinions on what to use. The default answer will always be get the best you can possibly afford. Recording audio is not that taxing...just about anything will work ok as long as you optimize your OS.
Case in point. My recording rig is composed of a cheap HP laptop running Vista Home (barf) from WalMart. I bought it during one of their Black Friday sales for $285. It has a 2.6ghz AMD Sempron (AMD cheapo Celeron clone), 2 gigs of RAM, and a 160 gb hard drive that operates at only 5400 rpm....it's a far from optimal rig. I use
a ZOOM R-16 USB 8x8 interface which is a bit of a CPU hog. By all accounting, this should be a horrible system. Truth be known, it works flawlessly. Once I killed all the uneccessary tasks running in the background via the system configuration utility (Services and Startup), the CPU hit was literally 1/2 what it was running in it's default configuration.
I record using SONAR 8, but I'm considering a jump to Reaper due to it's ridiculously small CPU footprint.
All that being said, I do not run any softsynth plugs. Were I to do that, I suspect my CPU would not be up to snuff and if you intend to go down that path, buy something with a better CPU, dual-core at a minimum.
Good luck and let us know what you get and what DAW you'll be running.