Well... that's a pretty general question. More helpful answers will come with more specifics about what you want to do.
To settle the Mac/PC question I'd figure out what software you want to use. Steinberg (Cubase & Nuendo) and Pro Tools run on both. Cakewalk products and n-track are PC only. Logic & Digital Performer are Mac only. FWIW, I tend to favor dual platform programs... no use locking into a platform as well as a program IMO.
Next up is the interface. Pick whichever meets your i/o, resolution and budget needs.
Once you know what program and interfce you want, you can use its requirements to spec out a machine to run it with. In addition to the Alienware PC's, Sweetwater makes "Creation Station" PC's- I don't know much about them but I DO know that having a QUIET machine is rare and wonderful. Off the shelf PC's don't assure you that, though I've been impressed with the recent Dell desktops I've run into recently in that regard.
For Mac- if you can afford the Dual 2G or above its worth it. They are liquid cooled and almost silent. Pay CLOSE attention to your software's compatibility requirements- there are some new dual core powermacs coming out that may or may not work with one interface or another.
Take care,
Chris