I have both the tyan board & the P3V4X. The tyan board is more stable, but the Asus is easier to overclock. My P3 550 will do 144mhz on the P3V4X, but will only go 133mhz on the Tyan. My old celeron 400 would do 450 in the Asus board, but wound ont O'clock at all on the tyan board.
My HP CD-RW is much happier in the Tyan Board than in the Asus one. I also think the Award Bios on the tyan board is easier to navigate thatn the one on the P3V4X. I have 3 friends who are using the tyan board (built two of the systems myself) without complaints. For someone who does not have too much experience with motherboards/CMOS changes, the Tyan board is rather straight-forward & they have a very good users manual (much better than Asus').
To overclock the tyan board more than 10-15%, you'll need a slotkit that allows voltage adjustmenst since you can't do it in the BIOS. Also, the Asus board offers more BUS speed options (especially between 66mhz & 100mhz) than the Tyan board to 'fine tune' your overclock.
Either board will do you well. If you want to squeeze every last mhz out of your CPU, go with the Asus. If you want a rock stable platform & are satisified with moderate overclockability, go with the Tyan board (Micron uses them in all their P3 computer systems, Micron would not have picked this board if it wasn't top quality).