I really only build dual socket 370 and dual slot 1 builds because these systems are still very usable for general browsing, video playback, adobe flash work, utube, CAD, etc.
For dual slot 1 boxes, I recommend a Dell Precision Workstation 410. It works with up to dual 850's fine and is based on the Intel 440BX chipset. The Tyan S1598C may also make for a good substitute, but I haven't tested this yet. This Dell system has worked reliably since Dec. 1998 on a continually-on basis. I bought it new and customised from Dell back in Dec. 1998 for $4100 and was worth evey penny. It originally had 400 MHz chips in it, but I upgraded them a few years back to dual 850 MHz units for next to nothing. I still use the integrated Ultra2-LVD SCSI with a Seagate 73 GB ultra320 drive. For storage and backup of personal files (including video and photo), I added 5 TB of SATA storage a few years ago. I have used it with an NVIDIA MX440 and FX5200 and am going to try to plop in an NVIDIA 6200 I got for $9. The Workstation 410 has my favourite case of all time - easy snap on/off and still solid and sexy looking. It holds 4 hardrives. The power supply is original and has never failed.
For dual socket 370, I personally recommend a relatively inexpensive Intel SAI2 motherboard. It houses the Intel ServerSet III LE chipset and natively supports Tualatin III-S 512KB 1.4GHz chips. It is a very stable board and comes with 2 PCI-X slots for fast SCSI/SATA and 1 GB ethernet. The only draw-back is no AGP slot, but a PCI NVIDIA 6200 works beautifully.
The other two dual socket 370's which seem to be good from the specifications are,
SuperMicro P3TDE6
Serverset III HE chipset
Built-in Ultra160 SCSI
2X AGP
6 PCI-X slots, 3.3V and 5.0V
Native support for Tualatin III-S 512KB 1.4GHz chips
SuperMicro P3TDDE
VIA Apollo Pro 266T
4X AGP
NO PCI-X
Native support for Tualatin III-S 512KB 1.4GHz chips
I've been meaning to get around to testing these boards, but would be reluctant to replace my existing Precision Workstation 410 or Intel SAI2 setup with these fancy boards because my existing machines are proven rock solid for what I use them for.
For non-dual socket 370's, I have two boards that I sourced after reading reviews some time ago. They are the Gigabyte GA-6VX7-4X (VIA Apollo Pro 133A chipset) and Asus TUSL2-C (Intel 815EP chipset). These two boards are popular with overclockers. The drawback of the 815EP is you can only use up to 512 MB of RAM. I beleive both boards support AGP 4X. I only sourced these boards for the purpose of an eventual VIA C3 build, which I plan on calling something like "The Last Cyrix", or "The Last Winchip." These are the only two boards I see any substantial benchmark results on using a VIA C3. I have two VIA 1.2 GHz chips I plan on putting in there.
Best of luck!
Plan your life wisely, you'll be dead before you know it.