The best Slot 1 motherboard (and PC) I ever owned was a Dell Optiplex something or other back in the late 90's. They were huge dual slot 1 server beasts (I had 3 of them) and each came equipped with dual 550Mhz Pentium III's (upgradable, but I never did or needed to), 512MB SD RAM, an AGP slot and around 5 PCI slots. The motherboards were huge and full of features. They had onboard SCSI controllers and each machine I had came with two Quantum Fireball 8GB SCSI drives, as well as a floppy drive and plenty of space for more hard disks. There was also a separate SCSI card in one of the PCI slots for extra drives and the onboard sound was some 32-bit Crystal Audio which sounded really nice, even compared to my Aureal Vortex in my main gaming rig.
The case opened out sideways like the door of a Ferrari and getting to the board and hardware was a piece of cake. They weighed a ton but were practically silent when running. Each machine I got had a Diamond Viper 770 (which I think was a TNT2 Ultra but I was never sure). I ended up giving one to my brother so he could make the jump to PC gaming, sold one to a guy in the local classifieds and kept one for well over a year. I ran it under Windows 2000 and threw a couple of VooDoo 2's in there for SLI Glide games.
I cannot remember the model number now, but I saw one (exactly the same but with a single 733Mhz Pentium III installed and a terminator card in the unoccupied second CPU slot) on eBay for £70. It was in perfect condition too. I made the guy an offer on it but he refused. This was only a year ago so they are still kicking about from time to time.
I'd advise going top end server grade Dell systems of that era. Back then Dell made some awesome beasts with their custom motherboards and top build quality. Search around on eBay and see what you can find. I had a similar full tower Dell server machine around that time, a Pentium 2 400Mhz system (only one CPU that time, I didn't add a second because I never needed to use the system) which weighed a ton but it was also a superbly made machine. Solid steel case ... I remember carrying it home from the office building around 2 miles from my house. I still feel the pain. Don't think I could do that now! I am old and weak.
I5-2500K @ 4.0Ghz + R9 290 + 8GB DDR3 1333 :: I3-540 @ 4.2 GHZ + 6870 4GB DDR3 2000 :: E6300 @ 2.7 GHZ + 1950XTX 2GB DDR2 800 :: A64 3700 + 1950PRO AGP 2GB DDR400 :: K63+ @ 550MHZ + V2 SLI 256 PC133:: P200 + MYSTIQUE / 3Dfx 128 PC66