btw, it might have onboard sound. I don't know why it would have a speaker jack without a sound card. Probably not the best but probably good enought for a mame machine. But USB would be your next best option. Other then that, you might not be able to get a sound card for what you can get a new motherboard for... fact of life with non-consumer grade hardware.
Also, with a $22 keyboard encoder for keywiz... keyboard hacks become more and more annoying. Consider space as your #1 issue with a bartop... and keyboard hacks usually make a LOT of clutter. the $22 keywiz + a IDE cable and your set with all your cabling needs in a VERY small space.
Anyway, you will probably want to take some time to draw out some rough sketchs.. And figure out your space.. Then decide on the joysticks, ect. There are some pretty low profile ball tops on ebay that switch between 2/4/8 that might be perfect for you. Also gives the classic red balltop.
example one I'm talking about... I don't know the seller and you should be able to get them for less..
eBay Link