I always understood that MAME didn't rely on a souped-up video card as the purist dev's didn't consider this true emulation?
You're right on one part of your statement, but incorrect on another.
Mame doesn't rely on a souped-up video for emulation, true.
HOWEVER, mame can and does use a high end video for converting from the original resolution to the PC monitor res, adding bezel art, adding scanlines, buffering screens, etc. Also, the default settings have changed: direct3D is default instead of directDraw, directX 9.0 instead of direct 8.0, switchres is off instead of on, ect. Each of these changes favor semi-modern video cards over old cards.
So step 1 could have been an extensive list of settings to change that differs depending on the exact old video card and it's weaknesses and version of mame and monitor type & res abilities and games trying to play (among other things) that might not help enough (especially with the system the OP has), or the simpler "Upgrade video card". [shrug]
As Havok hinted at, a P3 1 Ghz should be enough for pacman if you had a decent video chip set (and maybe more memory and maybe some changes from default settings) with current mame. Otherwise, step 3 or step 2.