As for fan noise, try a silent PSU and a blower type CPU fan - Pentium's help too.
This thread is probably old and long past dead but hey - I couldnt not respond to this.
Main sources of noise in a computer are PSU fans, CPU fans and video card fans (although for most MAME boxes this probably isn't an issue). There are quiet PSU's on the market as Minwah suggested. The CPU itself does not make any noise, therefore it will not matter whether you have a Pentium or AMD chip - it is only the fan that makes the noise.
Fans are generic and there are a lot of quiet models available. I have a low-speed 120mm fan on my A64 and it is virtually silent (not to mention kicking the crap out of P4's in game performance

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