Remember what each part of the computer is really used for.
-The processor is used intensively for running plugins. If you want to run a lot of plugins, on multiple channels, you'll need a lot of processor.
-The hard drive is written to for recording audio. If you want to record audio at a high bit depth and sample rate, you'll need a fast hard drive. No less than a 7200 rpm IDE drive. 10,000 rpm SCSI drives are better, but more expensive.
-RAM is used to buffer reads and writes. During recording and mixing, the more of the project you can load into RAM, the faster it will operate (RAM seek time is much faster than that of a hard drive). In general, the more, the better. And the faster, the better.
A good friend of mine runs a Pentium II 450Mhz, with SCSI drives, and lightpipe connections to a Yamaha O2/R. At 27 physical mono tracks, his CPU usage is only 20%. But all of his mixing, etc. are external to the CPU. As soon as he put one stereo channel of multiband compression in (a plugin), CPU usage shot up to 70%.
Bottom line, it really depends on how you intend to use the system. But if you buy top of the line, you probably won't be let down.
-mg