you've got enugh processor power, but that's not all you need. A good video card will be necessary as well. I'm not saying you need a 64mb monster or anything like that, but a cheap 2 dollar integreated video card is going to give poor performance no matter what you do with it. Also do you have enough ram? Mk runs sluggish with 128 mb of ram, it's runs adequately with 256 and full speed with more. The memory isn't as much for the graphics as it is for the allocaiton of the sound processors (both on your sound card and the emulated chip that mame is intrepreting.) I've got a athlon 900mhz iwth 500+ megs of ram and a 32mb ati a-i-w 128 pci card. Mk Mk2 and mk3 run smooth as silk on my machine.
I hope that helps