Well, I've just done a quick test on the original beatmania (JA-B) and on my machine - WinXP, 512MB, AthlonXp 3200+, once booted it runs at 100% speed, no frameskip or anything, using only about 60% CPU time.
Not sure exactly how well it will scale down but, your 2000+ is around 40% slower, so you should be right on the limit of running it 100% with no frameskip. Thats a bog standard MAme build (0.79).
Someone else may be able to confirm?