I can't speak for MAME use specifically but anyone looking for a really cheap P4 motherboard with integrated sound/lan (but not video) might want to pick up a Shuttle AV49PN for $52.50...
http://www.mwave.com/mwave/viewspec.hmx?scriteria=BA19740Features...
Uses the new VIA PT800 chipset
Intel Pentium 4 400/533/800MHz FSB
DDR 400 (single channel) (2 slots - 2GB max)
AGP 8X
4 PCI slots
8 USB 2.0 ports (4 onboard, 4 via cable)
onboard 10/100 Lan
onboard sound
Serial ATA with RAID 0,1
The board seems to be very well made and is quite compact in size.
Also of note is the fact that one of the PCI slots (slot 4) does not share IRQ lines with any other slot or onboard device. This is exceedingly rare in modern motherboards. That slot would be ideal for tempermental PCI cards like the SoundBlaster Live/Audigy line that don't like to share.
I just purchased one of these boards and paired it with an old 1.6GHz (400fsb) P4 and DDR266 memory that I had left over from another computer upgrade. Then in the future, if I desire, I can upgrade to more modern technology (a 800FSB processor and DDR400 memory) when the prices come down.
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Krick