Well, I made my post from work, and it turns out I did overestimate a bit - I'm at home now, so I'll give you the REAL skinny:
Intel P3 - 996 MHz
512 Meg of RAM
Win XP - Pro, SP 2
Sonar 4 - Producer Edition
Example Project 1:
18 tracks total
8 tracks = 4 softsynths (MIDI and Audio track each)
FL Studio w/ 14 internal drum/sample tracks
StringTheory
Korg Legacy Polysix
Korg Legacy Wavestation
8 standard audio tracks
4 MIDI tracks
13 VST effects (mostly Sonitus EQs, compressors and reverbs) (also, there are a few more out there, but they're on frozen tracks, so I don't count them)
This project is at its limit - it chugs along fine until the 4th softsynth comes in 3/4 through the song - at that point it hiccups, and drops softsynths. This only started after adding the most recent, umm, maybe 2 audio tracks and 4 effects? Before that it could handle the whole shebang.
Example Project 2:
38 tracks total
4 tracks = 2 softsynths (FL Studio again, and Korg LegacyCell - currently "frozen")
6 MIDI tracks
28 Audio tracks
13 active (non-frozen) effects.
This one runs fine all the way through, but only after freezing the LegacyCell (which is a REAL CPU hog) and a couple of audio tracks. At most, 16 audio tracks are "playing" (by which I mean there is audio data present) at a time.
So, I did overstate my case, but the most sensitive factor SEEMS to be CPU (which you have no lack of), and I think this is pretty good performance for the minimum requirements.
EDIT: Oh, and I'm using a Lexicon Omega (USB 1.1) for audio, and
MOTU micro lite for MIDI.