You’re talking a lot of dependencies here.
I’m having no problems with Apple’s Motion and Final Cut Studio in 720P HD with five layers off a single 7200 RPM drive and lightly compressed video on a top-of-the-line dual quad core Mac Pro with 8GB of system RAM and nothing else running.
But I don’t expect 1 stream of uncompressed HD video without a decent SATA or SAS RAID array on the system because one 7200 RPM hard drive just doesn’t have the ability to saturate the SATA channel like an array does.
Premiere Pro will have the same limitations. If you are working in HDV (which is really compressed) and you’re doing several streams on a really new system, you should be fine. But there is a big difference between Core2Duo and Quad Core and dual quad. Also your GPU comes into play with the more modern editing software (though I don’t think Premiere Pro CS4 takes advantage of a GPU unless you have an I/O card with a hardware codec (which is, in itself a GPU) doing the heavy lifting.
If you are looking to purchase, buy as much processor as you can and put at least 8GB (in a 64-bit system) of RAM in it. Get a fast hardware RAID controller card and RAID 0 or 5 stripe your array (you don’t get speed out of RAID 5 until you have at least 4 drives in the array). Buy a very good GPU and understand it will come into its own when (if) Adobe releases CS5.
And remember: You’ll still have to render. It’s a fact of life.
What if there were no hypothetical questions?