In addition to trying out some of these suggestions, I think I'm going to try a side-by-side comparison. Once I have the 98SE install done, I'll yank the hard drive and install XP on another one (same mobo, ram, etc).
Not sure when I'll get around to it, but I'll post my results as an FYI for everyone.
As a "scientific control": running MAME and MAMEWAH, same processor, same RAM, same MAME roms tested. No unncessary services / apps will be installed or running (Office, Messenger, etc). Really stripped down OS, dedicated MAME pc.
Only difference between the two will be Win98SE and some MAME version that works nice on one and WinXP and MAME on another.
I'll let everyone know how it turns out.