I'm still partial to the Tyan Thunder K8WE for graphics work since it has real dual PCI-e x16 as opposed to just the connections but only x8 and x8 on the two different slots in dual mode.
If you aren't doing graphics work though, the Iwill is the way to go as it has PCI-e expansion slots and the Tyan doesn't.
Ahh the choices...
As for usage, if you're asking if you need it you probably don't. Seriously. I don't need mine (see sig) but man do I love it. (OK, I use it to its full potential occasionally)
As for the *best* I assume you mean fastest. Without more info that's all we can give you. The fastest would be the Iwill or Tyan dual PCI-e boards with Opteron 275s (265s would be awesome too, but not the fastest.) dual nVidia 6800 Ultras, and with all the memory slots occupied by 512Mb or 1Gig 2-3-2-6 registered Dimms. (See my signiture for my Ram - fastest settings I could find 4 months ago. Might be faster ones now. Remember they have to be registered/buffered though!
Put in some WD 10Krpm Raptor SATA HDDs in RAID0 or some SCSI 15Krpm HDDs in RAID0 and you're set.
Of course, this will all cost you around 5 grand.
Here's a thought... Tell us what programs you are wanting to use, and how you use your current computer. Most importantly, say why you think you want a dual processor computer, and be honest. It is just plain cool. There are other reasons too of course. I just think all reasons are valid. Also, about what you're willing to spend. This way we can give you meaningful feedback and *best* won't just mean fastest.
Iwill Zmax DP SFF Barebones kit... (300W PSU, 8 USB, 1 FW, etc...)
Dual AMD Opteron 275's | 2 Gigs (2 Dimms 1024Mb) Patriot RAM 3-3-3-8
WD Caviar 80 Gig HDD - System
WD Raptor 74 Gig HDD
ATI Radeon 9800 Pro 256
NEC ND-3520A Dual Layer DVD±RW | Mitsumi Floppy/Card Reader combo