I agree with Kiff. Building your own PC is the way to go. It's not hard at all. My mom did it once.
If you plan to record multiple track projects, you will need two hard drives. One for the OS and one for audio. Multitracking on a single drive machine will increase latency, cause audio dropouts, and ultimately make your drive head work much harder which will cause it to fail prematurely. If you go the laptop/Macbook/iMac route, multiple drives can be problematic depending on which model you choose. Many of the those machines only have a single hard drive and a single firewire port (removes the option of connecting an external drive because you will most likely use it for your audio interface). Mac Pro's can easily accommodate multiple drives, but they are expensive for what you get.
I've maintained an all digital studio for the last 12 years. I build my own Windows based machines. Very cost effective as compared to Mac options. I use Cubase, but there are tons of options as far as the recording platform is concerned. There is even a light version of Cubase called Cubase LE that is relatively inexpensive. Pro Tools is the industry standard. However, I'm seeing more and more high end rooms pop up with Nuendo (made by Steinberg who also makes Cubase) as the head-end.
The Apogee Duet stuff is pretty cool, from what I hear. However, there is a ton of stuff that will work on Evilbay that is very affordable. Like someone said above, just stay away from USB. Not enough throughput/bandwidth for multitracking.
Lot's of companies are bundling their inexpensive interfaces with software, which may be your best bet. For example:
The TC Electronic Desktop Konnekt 6 is a firewire interface that comes with a mic preamp and a free copy of Cubase LE software for $199 brand new. I think Presonus, Focusrite, and Tascam have similar product bundles.
I hope this helps.
