i dont know if that will fix your problem entirely, but upgrading the ram will definitely help with your mame performance. what version of windows are you running? 256mb of ram just isnt enough to properly run anything above win98. max that thing out..ram is cheap. i noticed a difference in performance in MK2 going from 512mb to 1gb in my old motherboard. (athlon xp 2500+). i have since upgraded again to a different board and 3ghz p4, with 2gb of ram, and once again, i noticed performance increases, especially with mk3 and umk3.
definitely go for the ram upgrade...i dont know what type of ram your pc takes...but if it maxes out a 1gb, then it is probably either two 512mb sticks of either PC100/PC133 SDRAM, or DDR. looking at newegg.com, PC100/133 is more expensive than DDR, and you can get 512mb sticks for about $32 each....DDR is about half that. so either way it is pretty cheap to max that thing out, which will make everything run better.
if it matters, i'm running mame .124