Duals is the way to go, with out a question, here are some hard numbers for ya
machine 1
p4 2.0 eng smp @ 2.8ghz
512 pc2100 ddr ecc ram
4 wd 7200 2meg cache 40 gigs hd in raid 0 for speed and storage
gf4 TI 4200 agp 128 meg ddr card
machine 2
dual p3 1.4 tullys
512 pc2100 ddr ecc ram
4 wd 7200 2meg cache 40 gigs hd in raid 0 for speed and storage
gf4 TI 4200 agp 128 meg ddr card
machine 3
Dual MP 2100 +
512 pc2100 ddr ecc ram
4 wd 7200 2meg cache 40 gigs hd in raid 0 for speed and storage
gf4 TI 4200 agp 128 meg ddr card
machine 4
Dual Xeon 2.6 400 fsb
512 pc2100 ddr ecc ram
4 wd 7200 2meg cache 40 gigs hd in raid 0 for speed and storage
gf4 TI 4200 agp 128 meg ddr card
all machine with winxp pro with sp1 and HT enabled on the xeon
all divix encoeding with Vidomi (with multi procs enabled) and the same version of Divix codex.
all working from the same 1 hour PVR file from my Hauppagge 250 PVR, enterprise episode i think ~ 950 megs in size, all using the source file localy, and writing locally.
machine 1
28 fps
machine 2
38 fps
machine 3
65 fps
machine 4
81 FPS
so you can see that the dullys stomp down hard on the single proc machine, and it is just no contest in favor of the DP machines