If 3D fighters are your thing, consider using different emus which focus on performance over accuracy. Try pSX for the Tekken games, Soul Edge and many others, or Zinc, or something ike that. I use pSX myself, which runs at full speed down at 1.6Ghz with no external graphics cards.
Still go for an upgrade, but don't go silly as many games will only improve with new MAME releases. It took ages for Killer Instinct to run acceptably....