for the prices these days, you might consider looking around (ebay ect) for a super cheap faster computer. You can get a MUCH better computer for less then 30 bucks these days... Your talking ancient there.
But as it comes... look for a mame version that was made around when your computer was 'current'. You loose a lot of games but mame was optimized for your system. v.36 is much faster then v.37 because of the color depth internal to mame changed. And around .57 or .56 or so there was another system wide slowdown.
But I wouldn't upgrade anything on that computer... NO money should go into it. You might find someone who has memory to fit it... but I wouldn't spend more then 1 or 2 bucks for extra ram... heck, they should give it to you if you know them.
So optimize EVERYTHING for your computer... DOS will help a bit. And optimized mame builds (fastmame) will help... But don't expect much on that system.